In lifetime of boating, I have spent "a few" nights aboard; about 60% without A/C.
Here is my take: You are not at the Marriot, you are camping. Though the humidity can get irritating at night the temps are not always bad ( this is VERY location dependent ). Anchored out "at the islands", there was usually a breeze at night, and since the sea temp was mid 70s, low 80's sleeping on the deck required at least a sheet... Anchored "back in da swamp", where there was less breeze and the water shallower and warmer, yes, it could be uncomfortable. So, yes, an A/C can make your life more comfortable. THe other side of the coin is that a genny can get to be a PITA ( ie: Honda on swim platform ). There are some risks ( discussed elsewhere. I won't belabotr the point ) So the comfort is not "pain free".
An internal generator, as far as I have seen, is a rare item in a 28fter. And you will not want to run it all night, just enought to cool and dry out the cabin, then button up. But w/ the internal genny you also get microwave and/or coffee maker.
But again, on a 28fter, you will run out of space quickly, and such items take up space.
I had a 27 fter for several years, never had a genny, and when considering all of the plusses and minuses, I preferred the no genny plan.
Clearly YMMV. And again, location is important.