Negative on Portsmouth. The ferry company DOES NOT have their act together and aren't reliable. Jerk offs forgot about us for 4 hours. Then the guy wanted a tip. From what I could tell, their ferry was in bad shape and their company was in bad shape. Don't really want to trust them...for anything really. Oh and they don't get my business again.
Island wise, as long as you are on the beach, all is well. Otherwise the mosquitoes will DESTROY YOU. I'd slap my arm and kill 15 of em...that bad.
However from what I can tell, Portsmouth is it for fishing. Nothing really else to do and you can get from one end to the other in 30 minutes. IMO, unless you are fishing, you can do a lot more at other places.