[Destroyer]

I don't recall seeing ... did you check for corrosion on all your cables and connections? I'm inclined to think that the problem is electrical rather than mechanical. Reason being you said that the engine turned over fine with all plugs removed. If there was a mechanical problem then the engine would bind, with or without the plugs being in. So if we eliminate the mechanical then we are left with the electrical. That makes the prime suspect corrosion... either in the connections or in the cable itself. Easy enough to check, just run a jumper cable from your battery to the starter and try to start it. If it still binds, then you can eliminate the cables as part of the problem and move on to the next check which should be either a weak battery or a bad starter. Again, easy to check. Just remove your trucks battery and use it to replace your boats battery, then try to start it again. If it works the problem is the battery, if not, then you probably have a starter problem. (If you have one, find out what the draw is for your starter, then connect an amp meter on the cable and see how many amps you're pulling. That should tell you right away if your starter is good or bad)