In Sydney I belonged to an aquatic center. It had an attached gym but I just used it to swim laps.
I'm much happier swimming laps than working out on the land. I usually set a time goal and do as many laps as I can in the time I have. Usually I swam for an hour but if I was pushed for time I did 30 minutes and if I had lots of time I swam 90 minutes and hit the spa for a while afterwards.

Swimming is much easier on the body than other exercises and it works most of your muscle groups as well as providing a great cardio workout.
If you swim you'll start out with the old men doing breast-stroke in the sow lane and need a break between each lap but in a month or so you'll move up a lane and reduce the amount of time you spend resting.
Keep a bottle of water at one end of the pool - lots of people seem to forget that you need to rehydrate when you swim even though you can't feel the sweat.