Lift would be best.

But it not possible then to be redundant again , at least 2 pumps, each has it's own battery. Is there power to the dock? Could you install some onboard chargers and hook up to 120? If not I'd throw on a couple solar chargers too just to get as much unattended time out of the batteries as possible. Could you hire a neighbor kid to check on the boat at least weekly or more often? Can you secure a small generator nearby to power some onboard chargers and have someone run it once a week? All depends on what is available at the dock and how much $$$ you want to spend.