If it is already wired with a 110V hoist or two, lights etc then a small gen set would be the simplest and easiest. If you were starting from scratch and only launching two jetskis Friday, then recovering them on Sunday,just turning on a couple of 15watt lights when needed, then a deep discharge battery, solar panel and 12V winch would probably do the job. On the rare time that you kill the battery you could carry a spare down and give it a boost....By the next weekend the system should be recharged.
