[tsubaki]

I agree with Blue!
Most engines have some voltage leakage, further adding to the problem is the wiring leaking even though everything is turned off.
Charging the batteries ahead of time, having a battery selector switch and connecting the bilge pump (fused of course) directly to the battery or at the selector switch will stop the majority of problems associated with batteries in the boat. When dry docked, pull the fuse to the bilge pump.
An easy test to see if the charging system of the engine is operating would be to take a reading at the battery terminal (usually around 12.5 volts) then crank the engine and while running again test the battery.
It will read a higher voltage if the engines charging system is working. Usually around 13 or 14 volts.

We keep a boat in the water 24/7/365 using this method and have to replace the cheap battery about every 3 years.