Another option to consider would be a 15W solar panel and charge controller ($200 last time I bought a kit from Northern Tool), along with an alarm panel battery mounted in the Cat. You can get a 12AH (amp-hour, I don't generally see batteries labeled for watt-hours), battery for $50 or so. It's a sealed gel unit and will handle normal discharge cycles easily (I've done similar arrangements for other purposes).
Setup a waterproof quick disconnect so that when the boat is docked you can connect the solar panel to charge the battery. (In my experience a 15W 12V panel is about 18" X 40" or so, too big to mount on your cat). Keep the battery basically for backup juice in case the batteries in your handheld VHF get low.
Or, even simpler, get a kit like this:
http://www.amazon.com/Midland-NTVP1-...QNHSSXP8N96PK7 and drop it in the charger when you're not using it.
For use on Winnipesaukee, I think that a quality handheld VHF will give you all the coverage you are going to need.