While I agree that stretching before and after a workout is an important part of overall conditioning, I don't agree that it should be done underwater.
In order to take the best advantage of the stretching the muscles should be warm (recently used) in order to help prevent over-stretching and injury. Most lap pool water is around 75-80 degrees (Fahrenheit) and the cold water will rapidly cool down your muscles.
Also, stretches should be held for at least ten seconds, and when a swimmer is holding their breath, it is very hard for most people to relax into the stretch and hold it.
In my opinion, it's a much better idea to simply get out of the pool and stretch on the pool deck.