Anyone who has scuba dived knows that the increase in pressure is extremely rapid as one descends and any dive is inherently risky. Hence the “buddy system”. Never dive alone. The slow descent into the cold and pitch black at the depth of the Titanic is a whole different order of magnitude and would be terrifying in and of itself. Submarining isn’t for amateurs.
Rapid compared to what? Water is incompressible so the increase in pressure by depth is uniform.