[tsubaki]

If your not opposed to installing a battery shut off switch this might solve the problems however be sure to connect the bilge pump at the battery (fused) if the boats in the water. Voltage could be leaking most anywhere, solonoid, fuse panel, light switch or even main feeder wire with cracked insulation. Be sure to get the one you need (one or two battery) and don't buy the cheapest one.