Yeah I know that Vaders suit could help him survive in the vacuum for a short amount of time, but would it work submerged under water?
After 15 minutes of assorted Googling, I was not able to find a single piece of canon evidence pointing either way to the suit being waterpoof or not.
However, we know that the suit IS hermetically sealable and able to with stand vacuum.
As Wookieepedia says (sourced from Empire of Dreams: The Story of the Star Wars Trilogy):
McQuarrie added a distinctive skull-like helmet for the character when he read the script of Star Wars Episode IV: A New Hope and he became aware of the fact that Vader would have to cross through the cold vacuum of space in order to board the captured Tantive IV at the start of the film [EpIV: ANH - DVK]
It is fairly likely that a suit that is hermetically sealable against vacuum would also be sealed against water; and at the very least proof against some water pressure.
Having said that, we don't know if the suit's externals have any electrical systems that are NOT waterproof and can short out.
More likely than not that shouldn't be the case as we know TGFFA robotics technology is easily able to build waterproof mechanical and electrical systems, for example R2-D2 was none the worse for wear when he went swimming on Dagobah.
But there's no unambiguous canon proof.