Hi there,
I have read up on this, as I will have my cat onboard soon too. Cats are natural swimmers, but the cold water can shock them. One suggestion is to nail a piece of carpet from the back deck that hangs into the water (assuming your cats have claws) they can climb up on their own, if they fall in. Another idea is to have a long handled net available (like one you use to scoop up a large fish), you can reach over the side and "net" your pet if they go in.
I also use a natural litter for the box made from pine shavings called "Feline Pine" and it is the best stuff I have found. It has a light pleasant odor and works like a charm. It can be scooped and small bits are water soluble (they can be flushed into land toilets, but I would not recommend flushing them in the head). It can be found in many grocery stores and at most pets shops:
http://www.petco.com/shop/product.as...cm_ite=5397109
Lastly, from what I have heard, the cats adjust well and their main complaint may be the noise from the engines. So, just make sure they feel safe until they adjust and you will all be comfortable.
A side note: the boat I bought used to have a cat living on it and the owners said she loved it and would even catch and bring home the occasional small crab or sardine!
Hope this helps.