One of the most oft used items in my tool box is a heavy duty magnet. Can't tell you how many times that sucker has gone down the keel or under an oil pan to fetch something I didn't have a shot at getting otherwise.

Also, some cheap insurance is one of the portable battery packs. You can get one that will start a small gas engine or most outboards for under $150. Lots of them have lights on them, and maybe a cigarette lighter-type power adapter. Great for use on the boat in the off-season. If you have a dead battery, it will likely get you started, but you could at least use it to power electronics to use the VHF (make the emergency call FIRST before you run down the battery pack!).

You can get a commercial grade one like they carry on tow trucks that will turn over most diesel engines for under $300.