STOP!!! I was on
Skomer in April for the first time and my main lens was a 70-300VR. This zoom range is ideal and there were alot of other photographers there with 70-200's and one chap with a 600mm f4 that didn't get used on the puffins at all!
The puffins are very habituated to humans and at the prime puffin spots you can get up to within a few inches of them. I even used my 10-20mm for a bunch of shots as they were so close.
A zoom range is necessary as they are flying all over the place and you can track them and zoom in for each shot. The longer reach of the 100-400 would be great for the other seabirds on the cliffs and the other spots on the island where you can't get as close to the birds.
The main point for viewing the puffins (in April) was The Wick. There was also a school of porpoises just off shore that we watched for almost an hour. There are no facilities on the island other than toilets and some accessories (water, binoculars for hire etc) at the drop off point.
BTW, if you can get to stay on the island I'd highly recommend it. When we were there the first boat left at 10am and you had to be on the 3pm boat back. This wasn't enough time at all and we missed loads of birds. Everyone we spoke to that stayed the night had seen peregrine, owls, sheatwaters, and a bunch more I cant remember. They could go out at dusk and first light and were much more relaxed compared to us as we were constantly watching the clock to make sure we could get back to the boat in time. We only saw 40% of the island!
Also, if the weather is even slightly windy with a northerly wind, the boat won't go as landing passengers can be tricky, so they don't take the risk. So have a backup plan and phone ahead the evening before to see if they are sailing or not. The Pembrokeshire coastal path is stunning and there are some gorgeous secluded beaches to explore.
Just my 2p worth!