Submerged crews

[Marcello]

One of the lookouts of U-99 described it as "the waves go over your head and you're in it".Lookouts wore safety belts thetered to the central column in the tower.
Though I suspect the dunking you see in the game is probably a byproduct of game mechanics rather than the actual behavior of the boat at what would be barely force 7.

[Tango589]

Ta for the info!

[GoldenRivet]

The watch crew on the bridge were frequently swamped by massive waves, dozens of boats even lost men overboard, some due to accidents, others due to enemy action, many more as a result of bad weather.

U-106 is a particularly bad case. On October 23rd 1941 while traveling on the surface in heavy seas, the outer hatch of the bridge was latched closed to prevent taking on water. When time came to relieve the watch, the second watch crew popped the hatch and went topside only to discover there was no watch crew; they had ALL been lost overboard in rough seas, rescue efforts were made impossible because of the poor weather and the fact that it was unknown as to exactly how long the watch crew had been missing.

[Tango589]

The life of a watch crew was not a happy one: cold and wet or baked in the relentless sun.