When trying to surface on water while swimming to regain air, the level you have to surface to to start regaining air isn't aligned with the level that the first-person camera stops being underwater

From rudimentary testing using teleportation, while not swimming, the height offset relative to the top of the water block where air gain begins is -0.02, and the "submerged level" (level at which the underwater fog effect is applied) is -0.131(1), which aligns with the water texture.

My suggestion would be to lower the level at which air starts regenerating to the same level at where the camera visually submerges underwater