Moistness is an in-game mechanic related to Oxygen and having to do with water. Moistness is an indicator of how long a creature can survive on land before they begin to take incremental damage.
Terrestrial and Aerial dinosaurs have no moistness since they live on land, with the exception of a few.
Fully Aquatic dinosaurs have a moistness meter of 30, meaning once that reaches 0 they take damage while still on land and if the humanoid root head is above the water level. Most fully aquatic creatures have oxygen as well, such as Mosasaurus.
Semi-aquatic dinosaurs have no moistness, but they do have increased Oxygen compared to their Terrestrial counterparts, and the ability to swim with no major speed loss.
At first, many semi-aquatic creatures also had a moistness bar, such as Sarcosuchus, but this has been removed from them.
The Bivalve Megalodon has the most moistness of any creature in the game, with 150 moistness.