Cats love walking gunwales. Despite their reputation for agility, they are not very good at turning round in place on a narrow ledge, or walking backwards. Most slips from gunwales seem to be the result of them changing their minds part way along and attempting one of the above difficult tricks.
When I've had cats on board, I've had a fisherman's landing net on the roof of the boat, ready to scoop a soggy cat out of the drink. As you've found, although they hate water, they can swim really well. The problem is not being able to climb out. Carpet tiles are cheap and can be used as a boarding net that claws work well on. Either hung off the boat, or on to the mooring. Each end and each side. When they get too manky, they can be replaced.
When they fall in it is, of course, your fault and they will use you as a towel to get dry. Aside from the water risk, I'd say boat living is safer for a cat than a normal house, as there isn't the road traffic hazard. Road safety, being another cat blind spot.
Sometimes a falling cat can seem to bounce off the water, barely getting wet at all. How they manage this I've no idea!
Jen