Evaporation in marine aquariums.

Evaporation is directly related to a variation in temps and how much humidity there is.
If there is high humidity, then there will be near no chance of evaporation.
The act of evaporation creates a cooling affect by removing the heat energy from not just the water, but from anywhere experiencing evaporation.
Evaporation is the way in which water can change from a liquid to a gas and this achieved by molecules colliding and transferring energy to each other until some near the surface have enough heat energy to escape.
You only get humidity if the room is relatively sealed where your aquarium is stored.
If your water is 24c and your room temp is 30c you get quick evaporation, if your room is 15c and your waters are 24c you will get quick evaporation as well.
Just remember only hydrogen and oxygen evaporate, everything else including the salt stays behind.
The oxygen in water as part of its H2O make up can not be used for the breathing needs of reef life, that is another kind of oxygen.