I agree - one should first practice in relatively calm waters and slow speed.
The trick is to use the water force to rise up on top of the water - like a surfer or waterskier, not to be dragged through the water with your head down.
We did it for fun, not as a safety drill, but in the process we learned how to be dragged and how to maneuver while being pulled through the water by a chest harness. Jump off the rear quarter so you do not destroy the lifelines in the process. Once you learn how to drag in calmer conditions, the faster speed actually makes it easier to rise out of the water (and it is actually fun).
The key to proficiency in anything is practice and you cannot expect to handle a situation well if you have never experienced it. I am not advocating everyone to use a stretchy jackline and a long tether, but if you are reasonably fit, can remain calm or have fun with it, and practice maneuvering, it is one approach to working freely and safely with a harness on.
If, on the other hand, you are not fit, or you panic in new situations, or try to grab the tether, or you do not understand the physics of it, then you are asking for trouble, and you should either stay in the cockpit, or maneuver slowly on a short tether.
I also believe you should practice all the maneuvers you may have to perform if all comfort and convenience systems fail in the worst weather, so you should be skilled at maneuvering around the mast and foredeck using handholds and be able to handle your sails yourself without any help in difficult conditions (if you stick with hank-on jibs and halyards at the mast as we poor/frugal/old school/purist sailors do, or if you have spent some time racing as foredeck crew/bowman, you necessarily gain a lot of practice at these maneuvers in less-than-ideal conditions).
Last edited by jameswilson29; 12-02-2011 at 12:35 PM.