My cat has been living with me on my boat for almost 2 years and has only "jumped ship" twice. Once, out of curiosity and once, by accident. When I brought the cat aboard, I purchased a heavy duty fish net...the kind with the long, metal handle, for "emergencies", like cat overboard. When he first went over, he was attempting to jump from the transom to the inflatable dinghy
I was dragging behind. I had just anchored and he leaped from the transom, hit the side of the dinghy
, and since it was not hard, the cat bounced off the dinghy
, into the back of the boat, and then into the water. I had been watching him and was ready with the fish net and simply reached over and scooped him up. When he went under, he immediately resurfaced, as cats are good swimmers, they just don't LIKE to swim. Nothing was wounded other than his pride and he never again tried to jump into the dinghy
So, I would highly suggest buying a large fish net with a lond handle. It may come in handy. however, if you let the cat run the decks at night, which they love to do, then you need something hung over the side of the boat that the cat can get it's claws into so he can get himself aboard, in case you are alseep and don't hear him. I use a piece of very thin perforated Rubber like material, similiar to the mats you can get to set things on inside so they don't slide around, except it's long enough to secure one end to the rail and have the other end touching the water. I've seen some people use pieces of carpet. Basically anything that they can claw into and that will hold their weight will work and you need to train them to use it, of course, so if the time ever somes, they will know what to do.
Lastly, I highly reccomed not just a litterbox, but the ones that have a "lid" and look like a carrier with a "doggie door" on the front. This has been great! Not only does it give the cat privacy, it keeps the odor inside the box, and when sailing, it does not turn over and spill when the boat leans. Also, something the cat can scratch on is helpful so that they don't make scratching posts out of your upolstery! Good Luck!