I would wire the boatup just like you plan.

Next trip out [can you run it in the driveway?]leave the fuse out on the sonar unit.Take a meter with you and connect it to the two poles of the batery.Check the voltage at the battery with the engine runnning at differant rpms.I am not amiliar with the charging system specifics on an outbourd but I would suspect that your meter should not show any voltages higher than 14.5VDC.Idealy around 12.5vdc-13.vdc.Anything above14.5vdc would indicate a bad regulator.

I am no expert this is just what I would do.