If your boat does not have access to shorepower, get a small solar panel. I dont think you will need the $300 version, but take a look at what fits your needs.
Depends how big your battery bank is, how sunny your cruising grounds are, and how long you cruise for.
The way I thought about it is as follows:
If you have a small panel (like 15 watts - a little trickle-charger) then you basically need to have a battery bank that can keep up with your cruising needs for as long as you plan to be on board, because the input from the solar panel will be lower than your draw. (I'm talking here about a boat with a tiller pilot, basic electronics, and normal use of a couple cabin lights.)
For my 26 footer, I get about 36 hours of underway time from my 220ah battery bank. That's non-stop tillerpilot, electronics, and lights at night. If that were my longest trip, I'd be fine with a smaller panel, because I could then leave the boat on the hook, and allow a small battery to recharge it over a week until next use. But I wanted the ability to take a weeklong trip - maybe an overnight sail, and then stay aboard for a week gunkholing around at the destination. This requires me to be able to charge as fast as I consume electricity. So I needed a panel that's larger. With my 40 watt panel, I don't get any dip in my voltage during a sunny day sailing with the autopilot on. On a long cloudy day, my voltage might drop to 80%, but it only takes a couple hours of sun to get that back up to 90-something percent, and a day to completely top-off the bank. On a 2-day trip from Mass to Downeast ME last spring, with a couple hours of sun during the dreadfully slow 2-day, 1-night delivery, the batteries were down to about 12.1 volts (as low as I'd care to go). As soon as we got some sun on the last day, the batteries started climbing back up, and the day after we finished the trip, the charge was back up to full. I consider this my worst case scenario - 2 days straight tillerpilot use, with no sun - and the system was at it's limit, but worked.
I think I paid $300 for my panel, and about $40 for my charge controller. Money well-spent when I consider that it saves me from having to use gas, producing smell and noise, just to charge up.
Nate