It is a super-scientific fact (I think) that swimming laps is one of the best forms of exercise for the body. Unfortunately, some folks can't afford Olympic-size swimming pools in their backyard and are forced to work with the smaller varieties, which are impossible for lap-swimming.
Enter the ExerSwim. This is a device that can be rolled into the pool and will create a constant current that is great for swimming against. Think of it like an underwater treadmill. Although, you may be better off buying enough land and installing an Olympic-size swimming pool, because the ExerSwim carries a hefty $4,000 price tag.