Well, there are only really two types of touch screen: capacitive and resistive.
Capacitive wouldn't work under water, and they're more expensive anyways, which leaves you with resistive. You can pick up a small resistive touch screen (NDS) for ~5 bucks shipped. If you assume this is pure (or relatively pure) water, then pressure increases at ~.44 psi per foot. That means that the NDS touch screen would have ~2.2 psi on it. I assume that would probably render it useless (as the water would press the screen for you!).
Not to mention that as the touch screen is about 4.3 in2, you have a total of ~9.5 pounds of force on the screen. Easily enough to break it in half unless the submersible is well pressurized.