I don't think a water alarm will help, if no one is there to hear it.
My boat has a high water alarm, but its for when you are sleeping on board, or relaxing during the day. I don't think it would help if no-one was on board.
Thanks for the web cam link, if I remember correctly, the larger boats are more to the left of where the web cam can see. Or perhaps they have it out of there by now?
Hopefully I have the location right, that was they told me as his mom can see all the activity from her front yard.