A few rolls of underground romex and a rented trencher would get you power to the dock. Beat the heck out of toting batteries.
Might need something heavier for that long of a run, but power to the dock would be my 1st choice.

Possible you could invest in a small 2000 watt inverter generator and nice 50 foot 12 ga cord?

Easier to schlep a small generator than disconnecting batteries and you would have a generator for other possible needs.

Just spit balling ideas.