I'm not very familiar with boats, but does it have a fuse box or at least individual inline fuses or a contact strip. What I am looking for is a place to disconnect the individual power users. Should that exist...disconnect one item at a time (per day) a see if the draining of the battery stops. If it drains over night, disconnect something else every day until you find the circuit that is giving you fits. You might want to mark the items that you are disconnecting so that it can all be hooked back up correctly afterwards.

This process can be sped up if you have a multi-meter or a test light for checking electrical circuits.

Is there any way to disconnect one circuit at a time?

Fish on!,
Noel