...but sometimes doesnt do anything at all.

When you turn the key on and nothing happens, do the gauges come on. Do you have ignition power, just no starter.
Or do you have no power.

Start with the battery terminals, wiring and ground terminals, then look at the terminals on the starter solenoid. Sometimes the drain for the exhaust manifold will leak right above the terminals for the solenoid and make a rusted mess of them. I had this problem about 8 years ago, a new solenoid and some gasket sealer on the threads of the manifold drain plug cured it. I also put marine grease on the terminals after installing the wiring, to keep moisture out of the connection. Check the terminals and connections on the ignition switch as well. Its possible for them to get corroded or get loose. Last spring I had an issue where the whole ignition circuit was intermittently not cranking, and no power to gauges. It turned out to be oxidation on the old glass and metal fuse. The fuse had continuity from end to end but the oxidation prevented current from passing through it. New fuse and back to normal. Many boats come with sub standard battery cables and terminals. On a boat that age, I'd replace the positive and negative cables with tinned Anchor marine cable and have a marine shop crimp on marine quality battery clamps. Do not use wing nuts on the terminals to retain the battery cables on the battery. They get loose and then you have the no crank issue.