rsub, for heaven's sake, please do not ever consider doing statics alone in water - regardless if in a pool, tub, lake, or a sea. Always, always, always have a buddy close to you. He must do absolutely nothing else than surveying you closely, permanently, and continuously - best by keeping you by hand and sending you signals by clicking four fingers, and pulling you immediately out if you fail to response to a repeated click.
As Jome already wrote, when training statics, you can blackout anytime - depending on many conditions, so there is no safe limit. You can even blackout few seconds after starting the apnea, if you hyperventilated or packed too much. If you are under stress, not concentrated, nervous, or not perfectly healthy, it can happen easily that you hyperventilate without realizing it at all. Unlike what many think, hyperventilating is not necessarily forceful ventilation, and does not need to take minutes to have effect - it is any type of breathing deeper or faster than normally (leading to lowering the CO2 level).
So if you happen to blackout just at the beginning of your apnea, before a life guard or a friend watching you from distance realize there is something wrong with you, you may be already dead.
There is simply absolutely no point in doing wet static apnea alone - you cannot push to the limits, so such training has zero effect. Either do it dry or find a buddy. Better yet, find a club, of found one, if there is no in your location - it is much safer doing it in an organized environment with more people knowing what to do in case of a problem.
In France we have very strict rules we have to sign when entering a club. Not only we must never train alone, but there must be always an AIDA or FFESSM freediving instructor present, leading the training, and additionally the local pool management in Lyon requires that there must be also a certified lifeguard dedicated only and exclusively to watching us. It means not the main pool lifeguard, but really one that is familiar with freediving and stays involved, alert and very close to us all the time.
Besides it, we are required to make accident simulations, and I can tell you that even if you theoretically know what to do, it is completely different when something really happens. If you do not drill it regularly, you can very easily fail.
These precautions are wise, needed, and not at all expensive if you are well organized - in our club we pay yearly member fees less than 200€ for the professional life guard, the swimming pool (4 hours a week), diving insurance, FFESSM license, and covering the training costs of our instructors. Still there is enough money remaining for covering costs of participation on several competitions for those who are interested in competing.
