I have never tried under water cutting, I know there are several folks that use a water table and adjust the metal so that it just sits on top of the water. This will reduce the warpage on thinner materials and help disapate the smoke. Here is a portion of a reply (on this subject) that was posted on the plasmacutting forum on Yahoo a few days ago.
As I tried to state before…the non-high frequency air plasma torches as used on the Hypertherm Powermax systems can be used over a water table….but are not designed to be submerged. There are some users operating Powermax systems with the torch submerged…..and I understand that they seem to work well…..but the torch is not recommended to be used this way by Hypertherm. Some users do underwater cutting at their own risk.