Assume humans have the technology and motivation to genetically engineer humans capable of breathing underwater and on land. Also assume for this example mechanical technology is somewhat constrained to make the genetic options more favorable.
Would it be possible to engineer humans that could survive relatively unassisted on land and at extreme depths?
By this I'm referring up to 1km underwater though to depths of 10km (Marina trench) or even further down to 20-30km depths.
I'm aiming to design a society were a mixture of these modified and regular humans would live in an underwater biosphere and would go out to work together in teams of submarines/free divers.
I see four main problems with this.
Pressure at great depths. From my understanding that pressure would crush anything with air in it so regular lungs are out. A gillung as proposed in this question would probably be needed but I'm not sure what other changes humans would need to survive at deep depths.
Lack of oxygen. A human breathing with a modified gill system won't be taking in as much oxygen as an air breathing human. This might cause issues if attempting to work extended periods on low oxygen.
Cold. Deep depths are rather cold but don't seem outside of the range of human survivability. Bring some winter clothes, possibly have a heated section on subs for mermen to warm up.
Pressurizing time. For the mermen to move between the land and sea environment they would need to go through some sort of decompression chamber. If this process is too long/expensive/difficult it could cause problems for the society mixing between the two species.
Trying to find creative solutions to these problems or to find other problems I haven't thought of.