When you enter a submarine underneath the water does water enter in the submarine?

Yes and no - though water never enters the crew compartment, there are accesses known as escape trunks that are primarily designed for escape use by crews of a sunken submarine. However, they're also used for exit/entry by rescue divers/crew or SEAL teams. Each trunk has an outer and an inner hatch; the space between hatches is used to let water in to flood the trunk and to equalize the pressure to the surrounding sea pressure, in order to allow opening of the outer hatch.

The trunk is accessed from the pressure hull (crew compartment) via the inner hatch, and then the inner hatch is sealed. The trunk is then flooded until the pressure is equalized, and then the occupants exit. After the occupants have exited and closed the outer hatch, the trunk is emptied of water using high pressure air. Once it's purged of water, the air pressure inside the trunk is then equalized with the air pressure inside the crew compartment, so the inner hatch can be opened again and the process restarted.