Blue wires could be anything. Boats don't have a wiring standard. At the bow I would expect running and or docking or if the manufacture was nice they are auxiliary wires.

The say not to use starting batteries because the charging circuit can be noisy and impede the sonor return.
Richard is right about poor grounding.
Skeeter is thinking the right path with testing the battery. If it's old I would take it in to be tested. If it's too small you could have trouble too. But think about your car, everything turns off while its cranking too (although its wired that way).