I'm not sure about the cost, but I do know that most communities have a lot of team sports you can join, and I think they're usually relatively inexpensive. Volleyball, basketball, hockey, baseball... whatever you think you'd like most! A lot of gyms or similar organizations, like the YMCA have pools that have open public times, and almost always have a lap lane. You should be able to pay a couple bucks per visit, or get a limited pass for just the pool or whatever.
I would suggest dropping by a Community Center and check out their listings/postings to see what's available, and also stop by local gyms to get a price list from them too.
There are always free things to do, like going to the beach to swim, or playing frisbee in the park, but a lot of them are either unreliable, or rely on your own incentive to get something organized (but then again, I prefer team sports and organized activities, so you might like something different)