Could I increase the f value to it's maximum to reduce the aperture size (and let in less light), together with the decrease in ISO?.
Do you understand 'The Exposure Triangle'?
You have 3 'settings' on your camera; 1/ the Shutter Speed 2/ The Aperture or F-No 3/ The ISO 'Sensitivity'
For any scene, there's a certain intensity of light falling on your subject, being reflected into the lens of your camera, which has a 'meter' in it to measure that intensity.
In olden days you used to guesstimate the light intensity of your scene by eye, and probably used the f16 Sunny rule of thumb to decide what shutter and aperture settings to pick for whatever film speed (ISO) you had. Then came electronics and hand held meters that gave an EV or exposure value, and they usually had a neat little computator dial, you set to the ISO of the film you had loaded and then the measured EV, and could read off a range of corresponding f-numbers and shutter speeds that all gave the same 'Exposure' for that light.
More modern cameras had inbuilt 'Through Taking Lens' or TTL metering, and more sophisticated ones still had 'coupled' metering, that actually made one or other of the settings for you.
This explains a couple of the 'exposure modes' you might have spotted on the dial on your camera. Green Box, 'Full Auto' lets the cameras electrikery decide what aperture, shutter and ISO settings it's programmers think probably best for the EV the camera meters. Aperture Priority, leaves you to select the ISO, and the aperture you want, then the electrickery picks the shutter speed to balance those to the metered EV. Shutter Priority does the same thing, only you pick the shutter speed and the camera picks the balancing aperture.
In full 'Manual' (exposure control) mode, you get a hi-lo indicator in the view-finder or on the back screen, that tells you whether the aperture, shutter and ISO settings you have dialled in will result in over or under exposure, and leaves you to twiddle the settings to get the balance between them.
Which is a long way round way of saying... It DEPENDS..... what exposure mode you are using!
If you are on Shutter Priority mode, stopping the aperture down will just result in the camera stopping the shutter speed up, to balance the metered EV.
If you are on Full Manual mode, you have to balance the settings yourself... so yes, if you want to use a longer shutter speed you would probably need to drop the ISO, and or higher F-No to balance the exposure triangle,
B-U-T... where is the Exposure Value coming from... and if you are getting 'bright' images, is that close to 'right' to start with?
Inference of your query suggests its NOT... some-how you are getting over exposure, your 'light' images, or perhaps you are getting an over-exposure indicator flash up for the settings you have elected.
If you are trying to make settings manually, and are getting and ignoring an over exposure warning, then yes,you need to close down the aperture, and or drop the ISO to keep the balance (or find the balance) for the brightness of your scene.
If you are using a semi-auto mode like Shutter priority, then you may have to dialling a bit of exposure compensation, where the in-camera-meter is possibly being fooled a bit by things like a bright back-ground, and suggesting you need more exposure than you actually want.
Comes down to understanding the exposure triangle, and how that translates to your camera and the more or less coupled metering modes it has.
OTHERWISE... if you are exploring longer exposures, then you almost certainly need a tripod and remote release to keep the camera still and stop the subject streaking in the picture from the camera moving rather than subject movement.
The old rule of thumb to avoid motion blur was to keep the Shutter Speed over 1/Focal length of the lens. So if you were using a 50mm lens, you were advised to keep the shutter speed above 1/60th Seconds. If a zoom, say a 35-70, the advice was to keep the shutter over 1/LONGEST Focal length of the lens, cos it was still that big and had that much leverage over the camera regardless of the chosen zoom setting, so 1/125s. This advice is a bit thrown to pit these days, especially with Vibration Reduction mechanisms in the lens... but it aint far off! But, you have an 18-200mm lens.. even with VR, you 'aught' really be keeping the shutter speed up over 1/100th to avoid camera shake blurr when hand holding, so dropping to 1s or more for deliberate motion blurr? Get a tripod!
Worth mentioning, but most cameras 'settings' are made just before shutter release. There are cameras that will, in suitable metering mode, 'adjust' the exposure in real time whilst the shutter is open, and the immediate example of this is the old Olympus OM film cameras that had an Off the Film meter, that measured the light intensity falling on the film, and would, usually stretch, often interminably, the shutter speed after the shutter was released, if light levels changed. This made light painting and long exposures and things like stage photography their 'party piece'. But most cameras make their aperture and shutter settings before the shutter is opened, and that's what you get for however long the shutter is open, and regardless of any change in lighting. And pushing longer shutter speeds, so the more likely that things will change during exposure.
You then get into the arena, less pronounced now with digital, of 'Reciprocity failure; where at long exposures, the film, or sensor, doesn't respond at the expected ISO,
B~U~T... you mention milking water falls... good example there; if you meter the scene, lets say you get a good old fashioned F-16 sunny EV that suggests f16 and 1/100th second shutter. Thats fine as long as the shutter speed is that short. But, as you stretch the shutter, the water reflecting the ambient light is streaking down the frame, and reflecting umpety times onto the film or sensor before the shutter closes. Now you can chuck whatever setting suggestions the cameras TTL meter suggests to the wind, you are trying to capture highlights, so you may need settings to under expose the whole scene to stop the whole picture 'blowing' and turning your twinkling waterfall into a mush of milk, and/or you may need setting to over expose the water and stop the detail in the mid-tomes beneath blending into the shaddows.
Enter the 'Craft' and knowing when the camera isn't going to give you what you expect or want... and more,. knowing what to do about it!
This is why milky waterfalls have become something of a cliche and Soooooo over done. It used to be an academic exercise, in where the electrickery in a camera was unhelpful as well as in 'effect' exploiting the aperture and shutter controls to get something that isn't actually as real life. But still.
Bottom line... you need to understand the exposure triangle to balance your exposure, and you need to know what metering modes you have and which is most appropriate.
Expanding on the F-Number issue mentioned by Mariner. The 'aperture' is exactly that, a hole, inside the lens that light goes through. The F-Number is a ratio of the effective hole diameter to the focal length of the lens. Eg, is you have a 50mm focal length lens, with an aperture of 25mm diameter, then 50mm/25mm gives you F2. A bigger hole, that lets in more light, say 35mm, would give you 35/50 or f1.4. So the F-no gets SMALLER the bigger the hole. Smaller the hole, so the bigger the F-No, so same 50mm lens, an F-No of 22, would denote a hole diameter of 2&1/4mm 'effective' diameter. Note I say effective diameter... lens apertures are rarely round!
On the topic of buying more gear: I have said if you want to experiment with long exposures that a tripod is virtually essential; in fact at anything less than perhaps 1/30th of a second it will be, With a practised hand, I can with a 35mm lens hand hold sort of satisfactorily to about 1.4s... to the point that VR is essentially dong nothing for me, but I'd rarely go lower than about 1/30th, even at 18mm and a LOT more at longer focal lengths, HOWEVER! You 'may' improvise.
Basically you are using the tripod to hold the camera still on a support. That support doesn't have to be a set of legs with a neat screw into the bottom of the camera. It 'just' has to support the camera and stop you shaking it, I have, very often rested a camera on a park bench or dry stone wall or other convenient impromptu support, and leveled the aim with a scrunched up jumper or screwed up fag packet or whatever. THEN so I don't have to touch it to press the shutter, fired it on the self timer, built into the camera, when I haven;t had a convenient cable release or IR remote...
You DO NOT need to spend huge sums of money on photo-parafanalia.... you can get results with just a little know how and craftiness, like this. (I have been known to use a pair of sunglasses in front of a camera without filter thread in substitute for a grey-grad as well! )It's ALL in the know how and having enough to know what you DON'T need, as much as what you DO!
'