|Before connecting to a battery, I'd suggest drawing a full schematic of the existing wiring to sort out existing wires and potential current flows. Old boats are so often modified by previous owners. Not sure about the dangling wiring, or those 3 black wires. When we did our refit, I consolidated all the wiring, ending up with only one ground wire on the engine block leading back to the engine's main battery on/off switch, and only one ground wire from there to the engine start battery. We also have a ground bus for other circuits. Some alternators ground through their bolt point to the engine block, some have a dedicated ground wire, and if that black wire is DC ground, then it might possibly be part of another circuit, like the alternator's regulator. Hard to tell from here. Hope that helps a bit.
(Since we also have a house battery, we actually have a dual switch system to allow for emergency battery combining, but that's another matter.)
1974 Mark II C&C 27