Switch panels are really simple. Basically consisting of some form of positive bus, a switch, a fuse/breaker, and a negative bus. How you do these parts is up to you The hard part is providing sufficient protection. Electronics and sea water are not friends.
Personally, I am a fan of using MoSFETs to do the (high-side) switching, then controlling them from a logic-level (5v or 3.3v) supply, which may be manual switches or a microcontroller. This has a few advantages, it keeps heavy cables as short as possible, it removes physical contacts from the high load path, and it means you can make the panel smaller. It should also be easier to waterproof, and therefore last longer.
Incidentally, google Image search has lots of example switch-panels that you can draw inspiration from. Above all, keep it simple.