Northern gannets feed by plunge-diving, folding the wings back into a dart shape and entering the water at speed from heights that can reach roughly thirty metres. The impact is severe enough that the anatomy is visibly adapted for it rather than merely tolerant of it.
The external nostrils are closed over, so water is not forced into the nasal passages; the bird breathes through slits near the corner of the mouth. Subcutaneous air sacs over the breast, neck and face cushion the blow and help the bird resurface. Strong neck musculature and a sturdy skull absorb the deceleration, and the bill has no external hook that would catch on entry.
The dive itself does not catch the fish. Gannets carry momentum a few metres down and then swim with wings and feet in pursuit, often taking prey on the way back up, so dive depth is much greater than the entry alone would produce. The frequently quoted maximum speeds vary between sources, and the exact entry velocity of a wild gannet is not well pinned down.