Dometic - Cruise-Air used to make a portable sea water cooled unit packed into a Samsonite suitcase. 7,000 BTU, about 40lbs and very compact and almost silent ( I have one). You might find one second hand.
Alternatively get one of their units and build it in underneath the V-berth. More expensiive, but on a 28 ft boat space is a premium. Cruisair DX Self Contained Air Conditioning and Heating Systems - Marine Boat
You can get the same units cheaper on eBay (new).
I have a Honda EU1000i but I find it still too noisy at anchor (I hate to to this to people anchoring nearby). I made a noise proof box but then have problems with heat dissipation. I still find a wind scoop best out of the marina, or fans if there is no wind.