The water pressure is really the least of the problems. They train to escape from about 100m down but that is extremely shallow for the ocean. Much deeper and you're gonna have trouble getting back to the surface before you drown.
probably an air thing or two on the submarine that they could use to help with the issues of air
sharks, yes- I don’t think that there are other oceanic predators that they would deal with other than the shark
could even use the air deals to fend off the sharks however it’d be tough because you have 40 men and women who also need breathe/O2. although if the O2 is concentrated enough it may last longer
from hatch to surface with concentrated O2 thing should take roughly 35 seconds. multiply that by 40 and you’re at ~25 minutes give or take to account for hatch issues