There is other types as well, the ones that are built on that shatterproof glass they often use for kiosks and ticket machines, they work using light, i think, with emitters and sensors at the side of the glass that notice when there is more light bouncing back into the screen, and yes, they often are not very sensitive.
Also, resistive touch screens can work under water but capacitative ones cannot.
you are right, thanks for the input!
interesting that resistive touch screens even work completely under water, with your iphone screen you already notice that it doesn't work with just a few drops of rain!
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.