Can't help with the chestcold, but.
Ever thought about putting that big arse freezer on a trolley fitted with balloon wheels, that way you could pull it up the beach etc. OR, jimmy up a block and tackle, reduction pulley and yank it up. Something like those baggage trolleys at the airport ???
Further, and I know you've already thought of this. Dry ice, or in fact normal ice will hold fish in a good esky for up to 8 days. Stack the fish whole and upright.
Once you get back to the mainland, I am sure you can find a place to fillet and freeze the goodies. Again further. Cryovac. works really really well.
Ebay or 4 x 4 forums ( classifieds ). Other than that, a small 240V tuckerbox and a small gennie.
Good luck with all that........
cheers Phill