@Tdub i’m No boat expert, but I am an EE. I like the idea of checking the off current with your ammeter/multimeter provided current draw is low, which it probably is. Safer to test currrent with clamp on ammeter first but most of those are pretty high current devices and won’t give you enough resolution to pick up something small. Based on your symptoms I’m pretty sure you’ll see some current draw. At that point its a question of isolating circuits to find the culprit. If it still happens with the master off then It’s either something that bypasses the master, or perhaps some weird current leaking somewhere before the master. Maybe try to disconnect everything that bypasses the master such as the stereo and bilge suggestions. Do these have individual breakers/fuses? If none of these are causing the draw then you could try to follow the line to the master and look for connection points etc that might be a problem. One finally unlikely scenario is current flow across some surface. This really shouldn’t happen but if for example some conductive grease or fluid was on the surface of your battery, solenoid, or something else that’s always live you could have a small current leaking across. Very unlikely in a late model fresh water boat, but if you rule out other possibilities might be worth looking around for something like that. Just some ideas to think about.