This is a question which comes around regularly and I defer to many on here who have far more knowledge and experience than I do; however I will give my 'perception', because I have become dismayed at something our children are losing on a daily basis.
The OP's question, so far as I can infer, asks about the legal situation with regard publishing and selling photos made in a public place. There is a subtext of sorts in that the question of photographing children is raised.
This question has a straight forward answer but with contextual and behavioural complications.
In the main it is not illegal to make photos anywhere. There are many who argue the case for 'photographers' rights' but 'photographers', be them working professionals or hobbyists, do not officially exist as an entity with which to have rights. It is the absence of legislation prohibiting photography which allows everyone to make photographs. There are locations and situations where photography is prohibited, but these are specific locations, invariably due to national security and situations usually involving human behaviour.
If you are in a public place nobody can specifically stop you making photographs. There is no presumption for privacy if you are in public and that applies to both adults and children. An individual's behaviour, or a group behaviour does not in itself bar you from making photographs; if someone for no clear reason throws off their clothes, smears themselves with marmite and runs down the street yelling, 'you either love me or you don't', they will have no case to prevent you making a photo of their behaviour, naked or otherwise.
If however, you photograph a specific individual in the street and that person asks you to stop, you should be careful as at that point a line has been drawn. You might chose to ignore the request and carry on, but it is a fine line now between potentially legal and illegal.
You run the risk of being charged by the police with harassment or maybe a breach of the peace. My understanding is that you can't be charged per se with an act of photography but rather the former two offences. For harassment to stick, your subject should have asked you to stop several times and for your behaviour to make any reasonable person feel harassed. They should also be the subject. If they are simply one person among several or many in a street scene, harassment probably wouldn't be supported.
There is also some confusion over commercial and personal photography. The law doesn't differentiate. If you think about it, a photojournalist is a commercial photographer. A street photographer who goes on to sell prints, is a commercial photographer. A press photographer, is a commercial photographer. All the above by virtue of the fact that they sell their images. Actually, as an aside, the long winded debate over the terms professional and amateur could be diluted if we replaced 'professional' with 'commercial'. That would offer a clear distinction undeniably based on the 'job' factor.
Where the commercial aspect is applied, it usually applies to the equipment used. You can wander through Trafalgar Square as a 'commercial' photographer and make photos, however, you cannot use a tri-pod. Equally, a police officer can stop you using a tri-pod anywhere in public if it's considered you are causing an obstruction. In all these cases it is about the equipment used. Issues regarding any use of photographs are separate and if you are court up in a legal wrangle it will be in the civil courts. Remember though it is usually the publisher of the photo, not the photographer that is liable, if at all.
The question of indecent images again depends on the circumstances and is again misinterpreted. In public, photographing our marmite friend, even naked wouldn't be classed as indecent. Attempting to photograph up a woman's skirt would be classed as indecent, unless she had pulled the skirt up over her head. So, with all of this it's not the photography as such but the circumstances, you and your subject's behaviour that matters. An offence of voyeurism under the Sexual Offences Act can only apply if the subject had full reason to believe they were in a private situation, so photographing someone changing clothes behind a purposely screened area, albeit on the street would be potentially an offence, but not if they chose to change knowingly in full view of the public.
So, photographing people isn't illegal and you don't need their consent as such. But do you need a model release form? Well, no. In the UK, a model release has no legal standing. It may, in a dispute, help your case as it shows agreed intent with agreed terms but you don't need one. Don't confuse a legal requirement with terms laid down by third parties like stock agencies.
As I understand it, most of the above also applies to private property. Again, the law does not state with legislation that you can't make photographs on private property. The property owner can prohibit photography as a condition of allowing onto said property but it would not be illegal. The owner can ask you to stop and can ask you to leave; in the same capacity as asking you to not curse in front of the children or just expelling you because you have outstayed your welcome. Some force can be used to expel you but you camera cannot be confiscated or interfered with. Clearly this has the potential for both parties to break the law but most of these cases are civil cases. So when you go somewhere and it says photography is prohibited or restricted with signs or you are informed as a condition of entry it still isn't illegal.
I'll touch on the issue of photographing the police, armed forces and security personnel (by which I mean MI5/ GCHQ etc). Under Section 58a it is an offence to make photos of the aforementioned where the photos would be of use to a terrorist or for an act of terrorism. However this does not apply to making photos of these people in normal circumstances. You can photograph the police for example. If an officer is making an arrest in the street, walking around on the beat, just doing his/her daily thing, it is fine to make a photo. What you can't do is surveillance them going back and forth to work, documenting a behavioural pattern which could then be used against them criminally. Unless, presumably, you are an investigative journalist looking into corruption and building a case with evidence. It which case you could prove presumably prove your motives.
There have been cases where people have been stopped and questioned by the police for photography, where the police have used various (some now defunct like Section 44) legislation usually linked to anti-terrorism. The photography aspect has on the whole only been the catalyst, with some officers suggesting that photography is a suspicious act and could be associated with terrorism planning. This has been proven to be spurious and the last couple of years, barring a few curious exceptions, has been better with the police taking a more sensible approach. I hope I am right.
The issue of the sailing club comes under some of the above. The separation of photographing adults and children is to my knowledge unfounded. The sailing club can ban photography or set down whatever rules they like but I believe many clubs often mash up the legislation they quote. This makes things difficult to navigate when you factor in how you want to behave, your relationship with the club, with the members and your local reputation.
Often they will cite child protection but it is usually misunderstood and misused. We have been through the rather daft CRB debacle and had schools ban parents filming and photographing plays and sports days etc under supposed 'child protection'. Most of it is a load of rubbish and more people are starting to realise this now.
The OP's situation is an every day situation. The club are wrong to try an impose any sanction. Assuming the OP was on public land photographing a publicly viewed event the club have no complaint. Presumably the OP wasn't alone and other people were present no doubt making photos also. Just because the OP has approached the club with an offer to supply copies (charged or not) it does not alter the freedom he was under as detailed above.
There is no distinction between the adults and the children and the OP should think about whether to leave online the photos or not, for the reason I started this post.
There are two things I think are relevant. One is a massive double standard being perpetrated. The parent who complains you have included their child sailing, posts their own photos of their child or re-posts friends photos of their child....online. The clubs often do the same, they raise the issue then possibly post photos on their website. It is also a double standard to hold an event in a public place and then tell people they shouldn't make photos.
Secondly, people avoid photographing children in public. We see comments on here all the time, have done for years now and that means our children, when old, will wonder where they were. Go into Waterstones and you'll find social documentary photo books depicting kids in the street playing. We applaud these photos, some made by the greats, some by the likes of us mortals but so many of these fantastic images are hugely important.
We accept the fad for the mirrored selfie and grunt, hoping it will go away. We do unconsciously mentally scan images online and in magazines and adverts and newspapers that 30 years ago would never have been off the top shelf and say nothing. Teen clothing mail order catalogues depict kids in skimpy clothes, teen magazines push the boundaries hard.
Yet, kids playing in public, or sailing boats, are off limits if you listen to some people
I have tried to cover some of the legal side of things but I think we have a moral ethic to not avoid photographing people, adults and children, but to not shy away. When you set out to appease by removing or deleting photos immediately at any spurious request or demand, we lose something for the future and you perpetuate bad thinking.
I don't mean we should seek confrontation or to fuel an unhappy circumstance and I don't mean we should trample over people and ignore sensibilities or justified concerns, but we should, treading gently work to dispel poor understanding, perception and as someone else said paranoia.
We should be photographing normal, and the not so normal, everyday public life and that includes adults and children.
Matt