Assuming your inverter
also supplies your 240v outlets, then the circuit you have shown will also activate the relay when using the inverter, and unless you disconnect from shorepower manually, it will also connect inverter to shorepower, with obvious consequences.
Ive also used a DP relay, but this has a 12volt coil activated from the 12 volt inverter on/off switch. Disconnect the boat 240v outlet circuit (or whichever devices are to be powered by the inverter) from power source, and connect this only
to the relay moving contacts.Ensure there is no other connection to shorepower for this circuit. NC contacts are connected to shorepower live/neutral and NO to inverter 240v live/neutral. Inverter output is not connected to anything else
Existing shorepower connections to other circuits can remain as existing.
Both live and neutral are isolated between shorepower and inverter.
heaters etc are not powered by the inverter.
If shorepower is switched on with the inverter running, shorepower will supply the above only, however both power supplies remain isolated. (This is for a GRP boat only-not steel)
Care point is to ensure the relay is break before make
hope this helps