Does the alarm go off when you turn the key to Acces.? Does it stop after you start the engine? You should have a long constant tone to indicate that the alarm system is working properly when you turn the key to acc. In my old 454, this tone was constant until I started the boat. On my 496, it last about 5 seconds then turns off on it's own. This alarm should be present regardless of whether the engine is an inboard, I/O or outboard.
Is the alarm a constant tone, short beep, long beep, and if so, how many?