Get the biggest one you can find - my boat just has 1 battery so I got the smallest one just to "mantain" the battery. There is a large one that will actually "charge" a depleted battery.

I believe that they are good enough for a weekend warrior - let it charge all week and have it ready on Friday/Saturday.  The biggest benefit is that you don't have to plug the boat into an AC power source.

Also, another thing about them is they are not weather proof.  You'll have to make some type of enclosure w/ a plexiglass window to keep the rain off of the Solar Panel.  I used the original packaging for an enclosure - just use a lot of care when removing it from the package (don't destroy the package).

I could get some pictures of it tomorrow.