If using the dragon survival mod, Water dragons have an "inverted" oxygen system - they have limited air when on land. If Dragon Survival and Better Diving are installed, water dragons no longer have an oxygen meter when on land.

To Reproduce
Become a water dragon via Dragon Altar (Dragon Survival), note lack of oxygen meter when on land.

Versions
Better Diving: 1.3.4
Dragon Survival: 0.9.2
Forge: 36.2.20
Minecraft: 1.16.5

Fixed in v1.16.5-1.3.5