I spend at least an hour each time exercising and not just playing around. There is more to exercising in the pool than just boring laps. I use the whole pool; the deep and shallow end for my workout.
Treading Water works out the arms and legs and helps to build endurance. Sometimes, I'll tread water in the deep end for 1/2 hour to an hour at a time. I make sure to keep my legs moving, and I'll change things up by doing various arm exercises. You can do the arm exercises with out without weights.
I'll do bicep curls, and triceps kick backs till I can't do them anymore. You can work the pecs by placing your hands in a prayer position and then pushing your hands out and to the side while the hands and arms are underneath the water. Use you imagination, and just keep moving.
I would suggest starting off slow. Spend 15 minutes treading water and work your way up to an hour. I've found you don't have the same kind of pain signals as you do on land. You are pretty much weightless in the water and you don't feel resistance as much.
I over did it the first time and was in tremendous pain after I got out of the pool. The next time I worked out, I cut my time in half, and worked my self up. I can now do an hour each session, and I feel good when I get out of the pool.
Working out in the pool is easier on the joints, and is a great way to ease into an exercise routine. Being outside in the fresh air feels nice and the swimming pool is refreshing on a hot day.
Make sure to use sun block before swimming. Getting burnt to a crisp is no fun, and can lead to skin cancer. I know from personal experience!
I must also add, you should seek your physician's advice before starting an exercise program. If you don't know how to swim, it would be a good idea to get some lessons before wondering into the deep end of the pool.
I have a lot of other exercises I do in the pool, but I'll share those another time. Have fun this summer and stay safe!