I read about a couple that made a cat-tunnel out of screen/mosquito netting. It fitted tightly to the fore hatch and the created an outdoor sunroom so that the cat could be "outside" while still locked within the confines of the cabin. This was their solution to keeping the cat onboard while in harbour. I thought it sounded like a pretty good idea.
Even though I'm not a cat person, this blog entry sticks with me: Bika's Blog: Cruising with a cat