Many alternators on boats have external regulators. The field connection on the alternator may not be the best choice. Trace the wiring - the harness is usually color coded and the engine manual should identify the wire you need to tie into. Then verify it with a multimeter.

PS. IMO, a rectangular hour meter would fit in appearance wise better than a round one. Resetable digital ones are good for service intervals. I bought one but never got around to installing it.