I am sure there would be some depth limit for resistive sensors from water pressure, then you would have to resort to optical sensing systems, though, at that point you are probably too deep for just a scuba suit...
The issue with water is that it is electrostatically neutral, it has both positive and negative charge at the same time, because it is 'polar'. The capacitance effect for the touchscreen depends on on nonpolar materials, the same kinds of things that cause static electricity and such, and polar substances discharge the electrostatic charges. So it doesn't work, basically.