I wish there was someplace I could swim regularly. No Y near me and most of the fitness centers don't have pools. The park district offers the use of the local high school pool on Saturdays only in the AM.
I was a competitive swimmer for years. I started out swimming very young. So young that I don't even remember learning how to swim. I was always on several swim teams that allowed me to compete year round. In high school, our coach had us come in early before school and we would run, lift weights, and do some aerobics to cross train with swimming. It is indeed quite different- the land vs. water exercises. I definitely noticed joint pain (especially while running), but I am not a runner. Swimming is definitely a joint friendly exercise!
I agree with bee, it's going to take a bit of time to get your swimming fitness back. Like with anything, take it slow and it will come. Although I think I'm in pretty good shape, I wouldn't jump into an advanced yoga class because I just don't do yoga often.
Enjoy swimming!