Underwater vision is impaired because the density similarity between water and the eye's surface reduces light refraction compared to air.

“And when you're underwater, the the water essentially fills in the roundness around it. uh the air does of course above water as well but um but because of the similarity in the basically the density of the water and and the surface of the eye even though they're different you get less of a of a bending to of the light to a point.”