Is it possible to train 25 SCY underwaters safely?

After being "spoken to" by the lifeguards about training SDKs, I am wondering how dangerous it really is to do multiple full 25 SCY SDKs. Consider this set: Fins on. 10x/2:00 This is just an example. Basically I'm referring to any set that contains multiple full 25 SDKs on a fixed time interval. I've seen multiple people post sets like this in their blogs. I've heard that on some age group teams the coach will demand that swimmers complete N full 25 SDKs on some fixed interval or everyone does it over. The above observations would suggest that training full 25 SCY SDKs is a reasonable thing to do, but I've talked to some coaches and guards who seem to genuinely believe that even going past mid-pool underwater is just asking for trouble. For a reasonably fit masters or age-group swimmer (Let's say a "BB" or stronger swimmer between the ages of 10 and 70 who can comfortably train 4x1hr/week), what do you think: -Sets like these are generally safe as long as you don't do something stupid, like intentionally hyperventilate to the point of making yourself light headed before your push-off. -Sets like this are generally safe, but you can never know if you have an un-diagnosed medical condition that renders them very dangerous so you shouldn't do them. -Such sets are a little risky, but it's a risk you have to take to get really good at SDKs. -If you do this kind of training regularly, you will eventually pass out under water and possibly die. -The modern world is sufficiently rampant with litigation that no one can admit that sets like these are safe, even as anonymous vote on this forum.