You may want to try Kingston State Park just up 125, otherwise there was two tri groups, one based out of the Trek Store in Portsmouth which is now Colonial Bike and another in Exeter. Not sure if either club is still up and running but you can contact the Colonial Bike store and I'm sure they could tell you. They had some kits and sold wetsuits last time I was in so they may have a handle on it. Otherwise you may want to head south to Mass, try Trifury.com, they are a group out of Lowell, swim in Tyngsburrow, which would be about the same distance as trying to swim at Hampton or Rye beach. Otherwise, Kingston would be closest for you for a lake swim. Left NH a few years ago but grew up in Portsmouth...Good luck