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 damage.
Fully Aquatic dinosaurs have a moistness meter of 30, meaning once that reaches 0 they take damage while still on land or above the surface of the water. Most fully aquatic creatures have oxygen as well, such as Mosasaurus.
Semi-aquatic dinosaurs have no moistness, but they do have increased oxygen and the ability to swim with no major speed loss.