Use your voltmeter and see if there is a current discharge at the
voltmeter.


Or temporarily connect your VOM between the positive battery terminal and
the positive battery cable and check to see if there is any appreciable
current flow while the boat is in its stored condition, with everything
allegedly turned off. This will tell you if anything is drawing the battery
down while it is on the trailer or in the slip. Don't try and start the
engine with the VOM is in line with the battery cable. The operative word
was temporarily. Only just a test for static draw while at rest. If you do
notice any amperage being drawn in this situation, you will need to manually
disconnect each bit of electronics on the boat to find the offender.