The rush in immersion play comes from asphyxiation (also called breath play). When immersing a bound subject, special care needs to be taken since the subject has no good way of indicating distress. Safe-words can’t be used and if both arms and legs are tied it is very difficult if not impossible to get out of the water. To give the dominant partner some guidelines, take times how long the subject can comfortably hold his breath before play. Also measure the maximum time. Keep in mind that being tied and immersed adds stress. Stress causes your breathing to be more shallow and also increases the oxygen consumption. Therefore if can i.e. hold your breath for 75 seconds normally, under stress you may only be able to last 60 seconds.
Another consideration is the water temperature. Water that is too hot or cold can cause suddenly changes in your body. Cold water usually decreases your bodily functions and results in being able to conserve oxygen better. Hot water speeds up oxygen consumption. However, both too cold and hot can also help induce panic.

Unlike immersion play that deals with mostly still water, this form of play centers around running water. The type and amount of water used depend on the sensation that is supposed to be created.
Dripping water on a body part has long been said to be torturous, known under names like Chinese Water Torture. This form of torture has recently been reviewed by the MythBusters team. They had little success with this torture, showing that most of the stress associated with it actually comes from being restrained and the water has little to no effect.
Another form is torture with strong jets of water. Here the torture uses the pain of the water hitting the skin at high velocity. Just like with whippings the head is off limits. You should also avoid getting water inside the body through anus or vagina. This can cause internal injuries through to the pressure build up. Water temperature in this form of play is less important.