Started swimming about 3 months ago as some nagging injuries caused me to reduce my lifting. So far it's great, and the nice thing is that you get those kind of rapid gains you can only get when first starting a new exercise.
Usually I swim in a 25m pool, and occasionally in the ocean for a change. But I'm not good enough yet to get a real workout in the ocean like Kona described, without drowning myself.
I started out able to do around 500m, with lots of breaks in between laps. A mixture of crawl, backstroke and breaststroke. Now I'm up to about 2.5 km, with almost no breaks. I also work in interval training, like 10 or 20 sets of 50 or 100 meters at 75 - 90% effort, with 1 - 2 minute breaks between sets. Use kickboard or leg floats to improve form. I'm also planning on doing a few sessions with a trainer, since it's so form-based, and I never learned from a pro.
The good thing about swimming is that it's cardio and muscular training wrapped into one, and hits a ton of different muscles at once. So far I've been seeing modest widening and toning in the shoulders and back, strengthening in the quads and hamstrings, tightening in abs, and a lot more stamina when I play basketball or soccer. I supplement it with weight training once or twice a week, just basic stuff squats, deadlifts, barbell press, curls.
One thing is, I've lost about 5 pounds since I started swimming, which for me isn't a good thing, but with the right diet that shouldn't be a problem.
If anybody's been swimming for a long time, would be happy to hear your training regimen.