If it's a Volvo powered vessel, then you will likely need to purchase a "multi-link relay kit" in order to create a switched ignition source (Volvo pn# 21475508). The kit basically is Volvo multi-link cable which just plugs into an open port on the multi-link hub, and uses the power wires from the Hub to energize the hub of a relay to which you will connect your 12v house power and the load that needs switching.
You could just buy a multi-link cable and cut it, isolate the wires and create your own kit for less, but these kits are typically pretty affordable so not all that necessary to cheap out unless it's unavailable locally or in a timely manner.
The relay must be used because the current available from the Volvo harness is very limited and any excessive draw can/could/would cause communication errors and faults leading to loss of engine control.
