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Im pretty sure its not the battery because you had enough power to crank again and again...

yes. When the engine turn, the alternator produce all the current the boat demand (motor + accessories + recharge for the battery).

Probably not...
If the problem still ocur with the head unit turned off, your probably looking for a short circuit

Are you realy sure the alternator is top shape? A weak alterator can produce 20 amp at 2500 rpm an drop under 10 amp at idle... The engine need 16 amp to run if i remember...

oh... no amplifier, so i guess no big wire...

Just unplug the Head unit then...

amp = amplifier

Your sound system must be wired from the battery whit a big wire with a fuse on it. If you can find this wire, disconect it from the pole so it is offline...
Run the boat and see...

start by disconecting the amp(s) form the battery pole...

it is clear that the alternator produce less than the system demand at idle so start disconecting things while looking at the voltmeter and you should be able to find the over consuming component.