Battery charging batteries while stored?

[Blanco]

I've been thinking about keeping my batteries charged while the boat is parked & not in the water..

My friends tells me I can charge my batteries via a plug in charger to their house but I feel sorta guilty about using their parking spot where I have the boat & then on top of that plugging into their electric bill..

So I was thinking about just plugging two of these onto the batteries & placing them on top of my boat cover keeping batteries charged via the sun light. ( Free Power ).

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=44768

I would just adapt clamps to the ends to connect to the batteries.

I have one of these in my Mustang & Bronco that I don't drive much & they seem to be doing a good job.
The batteries always seem very strong now compared to before when I wasn't using these solar chargers..

What do you guys think?
& what do you do while the boat is sitting stored?

The other option that don't make me feel guilty would be to take my batteries home when the boats sits & place them in my garage hooked up to a trickle charger...

FYI, These are interstate Deep Cells.