What did it feel like to be submerged?

As long as the men were OK being in small space, it didn’t feel much different. They were inside the pressure hull, meaning that the pressure they felt personally wasn’t very different. It was sort of like being in a passenger plane. The change in pressure isn’t very noticeable. The difference, of course, is that in airplanes, the outside pressure is less. In submarines, the pressure was greater but it was still outside the pressure hull. In short, it was no big deal. Depending on the weather and the sea state topside, being submerged might have been preferable

The hull did compress a little when they went deep. However, at least on these WW2 boats, it wasn’t enough to notice a pressure change. It may still have been enough to harass the new crew members. Old salts could tie a string from one side of the hull to the other and pull it tight when on or near the surface. Then, as the boat went deeper and the hull compressed, the string sagged all out of proportion to the actual compression. It looked far more significant than it was.

Of course, the boat rides better as it gets below much of the wave action from above.