Answer:
277983.465 N
Step-by-step explanation:
 = Density of seawater =
 = Density of seawater = 

g = Acceleration due to gravity = 9.81 m/s²
h = Depth = 29 m
w = Force = 390 N
A = Area = 

Force in the system would be

The downward force must they exert on the hatch to open it is 277983.465 N