What is it that makes it dangerous/taxing? The impact or the pressure change?,(reply>), The deceleration. A lot of your inner organs are just loosely connected to the insides of your body. For example your brain is connected with some tissues but mostly just floating in your cerebral fluid which is supposed to soften the impacts. So if your body slows down from going 70 km/h to 0 km/h in the 3 meters of water, that is something your brain fluid has to handle. Concussion happen when the cerebral fluid cannot handle the impact and your brain gets hurt from hitting the inside of your skull. No idea how to train that, i guess the body has to get used to it, maybe you can strenghten your core muscles to provide additional support.