actually, in the summer I am at the pool several times every week. Part of my sugar control program. I do swim 50 meters under water and it is very "stressful" (pulse goes WAY up) but does not kill my practice time when I get back home.
Holding your breath is high tension for the entire breathing apparatus. The mechanism to hold the air in is also the same that causes "grunting" while playing. Swimming under water while holding your breath is surely not "bad", but that is not the same as "good". I prefer NOT holding my breath, rather letting the air out slowly like when snorkeling (which I also like to do - it is excellent for rhythmic breathing and I do have tunes in my head to support the breathing frequency).
I think that getting good data would require more than anyones (especially my) musing. I have in the past taken my blood pressure at the side of the pool after underwater, crawl, breaststroke and butterfly (I had a bout with high blood pressure due to sleep apnoe). Granted, it is only part of the story and not a fair comparison due to the differing efficiencies. Under water was by far the highest of the batch at 190/130.