I suggest getting portable A/C that runs on shorepower or a generator. We sleep on our boat a lot over the summer at marinas and got a 9000 btu/hr unit made by AMCOR (KF9000E). It has a small footprint and a neat automatic draining feature for the condensate. The flexible duct that blows out the hot air also has a small tube that mists out the condensate, so no need to drain.
I can tell you on the hottest, humid nights this thing will keep our small 292 cabin in the 60's! We just put up the full top (see pic), put the A/C in the step-down at the cabin entrance and blow it right in the cabin. Even has a remote control so you can adjust it without getting up. It is a bit heavy at around 50lbs, but has a handle and is pretty easy to get around. Overall I am very satisfied with it. We don't have a generator, so if camping off the beach it is back to roughing it with only a fan.