If you want to dedicate a starting battery to the engine and a deep cycle to the rest of the system, then you are going to need something that allows both batteries to be charged from the engine and yet only the starting battery to actually have power drawn for cranking.
I've never looked into that in a marine environment (I just use 2x marine N70ZZ batteries and dual battery switch), but in the 4WD community, that's very common. I believe Redarc make an isolator specifically for that purpose. You'd need to check that it's SAE-J1171 rated though...
To use that switch you have, you'll need two batteries the same, and wire the boat like this....
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