How do you know you don't have a starter or electrical problem? If you can start it with a jump, I'd think you don't have a starter problem, but maybe a battery, cable or ground problem. If the engine struggles when cranking, it could be a timing problem. Too much advance will ignite early and try to push the piston down the wrong way. If the engine cranks easily with the jump, the problem is in the boat electrical system.
Two common reasons for tight engines are bores-pistons-rings not properly sized or wrong bearings in a cheap overhaul. Owner assumes boars are worn oversize and buys over size rings, doesn't properly gap them and uses the old pistons. Same with the bearings, owner assumes crank is worn and buys oversize bearings or shims the existing bearings and now the clearance is too little. Unless you have receipts from a engine builder...
I'm suspicious of boats sold with an overhauled engine that has some minor unresolved problem. Often the owner did a poor rebuild, can't figure out what's wrong so sells the boat with some story.