You can wear the sensors in water no problem. The major problem that people find is that the adhesive starts to break down prematurely.
My absolutely favorite way of preserving the adhesive (all of the time) is using the STS shower covers. You can buy them in packs of 10. They came with the original, non-waterproof systems and create a watertight barrier around the sensor. This really prolongs the adhesive life and is easy to change once the shower cover adhesive wears off (5-6 days).
As for swimming with the receiver -- BE CAREFUL! If you get the receiver wet, Dexcom will not replace it free of charge anymore. I'm not sure what they charge for a replacement but I'm not sure if its worth it. It also causes one to be cautious in the bathroom as I know many who inadvertently drop it in the toilet (still not covered)!
I've always used swimming as a great opportunity to charge my receiver at home.