Hopefully you didn't try to start it last fall for an extended time with the sea water **** open... if you did, the engine may be hydro-locked (and may have sat full of water all winter)
If, as Barry above suggests, you can hand crank the moter, then with luck all you have is a voltage issue...
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"