Just to update this, got a reply from Richard at TecArc and he said the torches used on submerged cutting were specialised so it wouldn't work. Not sure in what way they are special but there was no elaboration.
Have also been doing a bit of searching and found some info from Jim Colt of Hypertherm, he mentions the HF torches should be fine but the Blowback torches are not the best for this. They will work but for how long is the question as they rely on the electrode moving to close the circuit and likely that would seize up through time. BTW the Trafimet torch I have is Blowback.
It may not all be a loss however as really my main aim is to get a better cut finish on Stainless. The cuts I get at the moment are pretty good but the cut is oxidised and black so requires a clean up and I was hoping cutting under water would negate this. I am thinking however that a water table with the water just lipping the top of the material would be fine and that when cutting the water would still be protecting the edge as the cut progresses.
Might do a few tests on some hand cutting and if successful then I may build a water table.