I've had interest in aquatics since I finally conquered the water (6th grade?). My research project in OT school was on aquatic therapy for CVA pts at a local rehab, and I also spent a fair amount of time lifeguarding. But the spur for this article is more personal... my dad had a heart attack 2 years ago at a young age and has been working hard at galvanizing the men around him to be proactive about heart health. My uncle has been resistant, saying that he can't do regular cardio due to his knee replacement... so here's some low impact cardio exercises you can do in a family sized pool. As always, consult a doctor before beginning an exercise regimen of any kind, especially if you have risk factors for cardiac disease. Seriously. Also, be careful in the heat, since that could aggravate a cardiac condition. Again with the checking with doctors.
Treading Water- an easy exercise that can be done for a long time. For variation, try arms only, legs only, or holding a weight above your head.
Walk Laps- good for a shallow pool, you can add resistance through leg bands, such as these from Speedo
Jumps/Skips/ Bobs- simple enough, more demanding than just walking. Jumps or skips in the shallow end, you can do bobs up and down in the deep end trying to get high into the air and deep to the bottom of the pool. Be careful not to over exert, especially if you are holding your breath.
Underwater Laps- build lung capacity by swimming progressively farther underwater. Slow your breathing during your rest breaks.
Side push ups- plant your hands on the pool wall and practice lifting yourself out of the pool. Be careful with this, keep your head back and away from the wall or your teeth are in peril. As an alternative, if you have a diving board, you can do pull ups while in the pool, you can also do these from starting blocks if your local pool has them and allows them to be used for this purpose.
Crunches- with your back to the pool wall, stretch your arms so they support you and let your feet dangle straight down. (this is 90* shoulder abduction for those who are anatomically inclined) Then, you can do oblique (side to side) crunches.
Stepping- if you have an aerobic stepping program that you are familiar with but have a hard time tolerating the impact level, weighted steps can be used in a shallow pool.
If you have access to a lap pool in your community, swimming laps is good cardiovascular exercise and can also help you work on expanding lung capacity and strengthening. To mix up the routine, consider using a buoy to force you to swim only with your arms, or a kickboard for the opposite. Also, many local community centers offer aqua aerobics... if you're already paying for membership, you can take advantage of the class.
ps- again, go to the doctor!! use these tips at your own risk.