My boat has two batteries, sounds like yours had one, and the wiring diagram shows only one. Mine are wired in parallel, not one per engine.
That probably explains why I have two different tie downs. The 2nd battery, left as you face the boat from the front (I'd call that the right or starboard battery) has a plastic box with a strap holding it down to two metal brackets. Looks like it should have a top, like a battery in an open runabout. Since it didn't have a top, and has no drain holes, it was full of water around the battery!
The engine and other grounds were connected to the battery in the original location, but the ground cable to the 2nd battery wasn't, so it wasn't doing any good, nor was it charging.