Hi Nic, I do commercial solar power projects for a living. You only need 2 things a charge controller and a solar panel. Don't bother trying to run solar to your charger, no point taking DC power converting it to AC then right back to DC, you would end up burning up more in the conversion than you would produce.
Look into the Morningstar Sunsaver: http://www.amazon.com/Morningstar-Su.../dp/B002MQSTQ2
I recommend you work with somebody like DC Power Systems to help you with the technical details: http://www.dcpower-systems.com/produ...?gid=44&pid=84
or maybe somebody at the Solar Living Institute can help you out: http://www.solarliving.org/
I might be able to help you a little but I don't know that much about off grid PV.
I can probably get you a 175 to 220 watt solar panel for about $400 to $500. Total system cost would be less than $600 installed and your power would be free for the rest of your boatlift's life!
shoot me a PM to: firstname.lastname@example.org
if you need help, i'll do my best to hook you up.