Does being under water provide any protection from a asteroid?
No. It depends on the severity. Water would be worse. If an asteroid hit in the ocean we would experience tsunamis the size we get from large magnitude earthquakes. The displacement of the water would be massive. It would also cause things like swimming pools sloshing around, common to the videos you see of hotels from the 2011 Japan earthquake. If it hit on the opposite side of the Earth, land would still be a better spot. Tsunamis can translate vast distances depending on the level of energy and surface deformation they create. Being in a submarine?  Again, no. A tsunamis can pass under boats. However, impact from an asteroid vs sea floor deformation from earthquakes are different functions. The submarine one, would require someone with a little more expertise.