I prefer to do as little as possible in the way of food preparation on the boat. I have a galley but it is pretty confining compared to kitchen at home! I do use the refrigerator on board but could make do with a decent sized cooler if need be. I generally marinate and cook boneless chicken breasts and thighs in quantity the day before we are leaving. If I were using a cooler I'd probably freeze half of them for the second day out. Sliced cold grilled steak is also good. Mayo-based dressings don't take heat well so I'd recommend pasta salads dressed with oil, vinegar and herbs go in the cooler as well. Hard boiled eggs in their shells can be used on their own, to dress up salads and to make egg salad sandwiches. For the latter, see if you can accumulate mayo in the single serving packets or just put the smallest jar you can buy in the cooler - I've thrown a lot of mayo away over the years! On top of everything in my hypothetical cooler I'd throw in a bag or two of salad and some bottled dressing.
I agree with Spike on his recommendation to get a grill. I had not seen the make he recommends (looks nice!) but make sure that whatever you get is sturdy and will mount securely. I would be very hesitant to have the grill just sit on the swim platform. Once you have a grill available then put steaks or chicken in a freezer bag (the regular plastic bags leak), add your favorite marinade and either deposit directly in the cooler or freeze them first.
Have a great time out there! And good eating!