I don't think you can just unbolt the battery from the alternator and start the boat. I think its circuitry requires the battery and alternator to be connected.

I will say I would replace the alternator, at idle it will put out 14V and at max output of 2000rpm it will put out 14V, the difference is the amps from idle to 2000rpms will increase.

Note - this is a reason if you jump start someone run your car to 2000rpm then have the try to start up this way you are putting out max Amperage.