If you wear a helmet with a decent Respiration enchantment, then at night you can see better underwater than you can in air. The ability to see better is great, but better than you can see in air? That seems over-generous.

In particular, you can see stars and the moon very clearly while under water, which looks very odd, and also surface mobs.

Apologies if this is a dupe; 'underwater' gives 170+ hits, but I don't think this particular side-effect of Respiration's enhanced vision has been raised.