If the boat is made of steel, why can't you use that for the ground?

I believe my two circuits (in my reference) meet all your requirements, consisting of multiple remote push-buttons with indicators that toggle a flip-flop controlling a relay. Have you looked at them?
Or do they seem too complicated to build for you?

 

Yes - to be honest Crutschow - your two circuits are beyond my capabailities to build. I looked for a "ready-made" solution after seeing them as it does look like this is the best solution.

The entire electrical system is isolated on a steel boat because of the risk of galvanic corrosion - there is a low voltage DC as well as a 230V AC (Ireland) supply on the boat.

Thanks Crutschow - I value your thoughts and help.