Dr. Awkward said:Immersion implies being surrounded on all sides by water.
I would agree that a more formal definition would be helpful, but, to me, immersion implies more than just being surrounded. I'm immersed if I'm in a volume of water and I'm completely below the surface. It's not just a layer adhering to my body.
Perhaps a better way to rule this is by how much water is covering you, in a manner analogous to fog and concealment. If you're just below the surface, you have cover. If your are at least five feet below the surface, you have total cover. (And, of course you could extrapolate this to other directions, if you are researching the "Protective Water Cube" spell.)
--Axe