How big is the boat?

If your main battery bank if only 15 feet away it sounds like a small boat. Do you really want to add the weight forward?

You will be running the engine anyway when using the windlass or bow thruster - you will want as much power as possible coming from the alternator. The instant you put load on that forward battery the voltage will sag and you will be drawing current from the main bank and alternator.

For so short a run you are better off with cable rated for the load back to your main battery bank.