Centre cockpit is the best type of boat and I just had the sand box on the aft deck. I sometimes used cat litter on long passages but ordinary sand is fine. I know someone whose cat is happy going on a pan of marbles.
I used a piece of fishing net over the side of the boat but if you have a dink in the water a piece over the side or back of that is good too.
DO NOT DECLAW THE CAT IT IS A DEATH SENTENCE FOR THE CAT.
Mine fell/jumped/ran off the boat about once a month.
DO NOT LET THEM LICK THEIR SELF DRY WHEN STILL SALTY. Give them a fresh water shower and a good dry first.