The alternator isn’t in the picture, so it’s a little hard to tell what’s actually charging the batteries when the engine is running. Best I can tell though is that when the engine key is on the solenoid parallels the house and start banks, so they should both have the same voltage when the engine is running. The OP doesn’t see the same voltage on both banks, so either:
- The solenoid failed
- The house battery select switch is set to off
- This boat isn’t wired as the diagram shows
- I’m misinterpreting the diagram
