Open water swimming makes things much more interesting if you want to do distance and enjoy it. No chlorine, no lanes, more fun.
Yes, my local place to swim is great – 600m long, rapids, strong currents (if you really want hard exercise you can swim from eddy to eddy upstream), and if you're lazy, you can swim the 600m downstream round the bend, get out and walk 250m and you're back at the start. Oh and if I'm organised in terms of working out where to get in/out and getting picked up / dumping gear at the other end, potentially there's another 20 miles of nice enough swimming before I'd hit the big river.
I've also swum in Snowdonia (some great mountain rivers there), loads in the sea, in the Thames (surprisingly nice to swim in), up near Chatsworth (amazing views and lovely relaxing swimming), Dove Dale (a bit of a walk for the swimming, but it's an amazingly beautiful place). There are loads of lakes that you can swim in too, if you're not into swimming with currents or want to be able to measure distance swum in a meaningful way (although lakes are a bit boring, I dunno why anyone would choose to swim in a lake if they had a nearby river).
The most surprising thing is how cold it isn't. Once you're in, the rivers are lovely and warm this year (since about June anyway). Although I do find if I go pool swimming it feels a bit too hot nowadays (and I resent paying money for it!).
Joe