We stayed at Mt Desert Narrows in May. Its a big park and its on a big bay that empties out at low tide then fills back up at high tide. It was beautiful.
There are many water view sites.
It was nearly empty in May but I dont know how busy it would be in Sept. The kids should be in school so maybe not too bad.
We dont care for the large destination resorts when they are full.
We didnt stay there but drove thru and the Bar Harbor KOA is on the ocean and seemed very nice.
You are correct about reviews; one mans meat...
We stayed at Bayleys in Scarborough based on rave reviews and couldn't get out of there fast enough. 600 sites filled with families who let their kids roam in packs un supervised.