Regulators vs. Conformers
There are two answers to the problem of an organism having to balance water:
Osmoregulators are animals that have to control their internal osmolarity (osmotic pressure) because their body fluids are not isoosmotic with the outside environment. So if an osmoregulator needs to release excess water, it does so; if it needs to take in more water, it does so as well.
This Mozambique Tilapia is an excellent
example of an osmoregulator.
Osmoconformers do not actively ( actively = by using up energy) adjust their internal osmolarity. This is because an osmoconformer's internal osmolarity is the same as that of its environment. This kind of water balance is available only to marine animals, which often live in water that has a very stable composition and hence have a very constant internal osmolarity.