Is there a cheap and easy solar charger available, that I can use to
keep the batteries full when I don't use the boat in a while? I just
want it to maintain the charge, so that I can be sure to return to
full batteries after a while of non-use, and don't have to deal with
shore power cable and charger. It needs to be able to sit outside, in
the cockpit.

There would be no load other than keeping the batteries (two, I think
54 Ah each) topped off.

Thanks!