Re: destroyed impeller
End plate. More work, perhaps, but finesse triumphs over brute force, engines can be clever about hiding stuff they've eeaten.
Denise, it would be simpe to take a magnetic reed switch (i.e. alarm window contact) and line it up on the intake seacock, but as the other guys said, there are times when yo might want that seacock closed. So, you'd be better off using it as a "seacock closed alarm" than a starter interrupt itself. One set of alarm contacts, one piezo buzzer, one relay, and of course one fuse. No biggie. $5 project, plus another $10 for the buzzer.