dunno where to suggest,
short or just slow drain ? try touching the pos batt leads to the battery 1 at a time and look for a spark. be aware of safety issues, fuel fumes etc, when doing this.
your outboards will drain batteries over time , esp if they have some age on them.
but why don't you put perkos on the outboards too ?
i have everything switched except 1 auto bilge , that way you can kill power to the whole boat.