We started taking Spike on the boat when he was 4 months old. He accepts it very well. We put a harness on him when underway and tether him to a padeye. Black cats also need a shady spot on sunny days. The biggest problem we have had is that Spike likes to wander around and often goes below when we are underway. That usually means that someone has to go down and release his tether and then reattach it when he comes back on deck. At anchor he has the freedom of the boat. He fell overboard once when he was a kitten. Fortunately he was tethered and I just hauled him back aboard. Since then he is much more careful and hasn't had a problem.
We haven't had any problems with sea sickness, but Spike doesn't like beating into head seas (he is a gentleman ). The only real problem is that he gets up EARLY and likes to have company - he makes sure everyone else is awake. He is an indoor/outdoor cat which may be the reason he gets restless in the morning. He is used to going outside at dawn. An indoor cat would likely be better.
We keep his litter box in the head. It is important to have a well ventilated spot for the litter box and to clean it out right away after significant us