Fresh water is the less concentrated of solutes than sea water and it (fresh water) tends to be less concentrated than the intracellular environment making cells to swell. Sea water, on the other hand, since it is very concentrated tends to dehydrate the cell.
The vacuoles of protozoans are internal structures specialized in water storage that when necessary liberate water to the cytoplasm. Vacuoles thus can dilute the cytoplasm for it to enter into osmotic equilibrium with the environment. Protozoans of fresh water then need vacuoles more since their intracellular is hypertonic in relation to the exterior. Without the dilution mechanism provided by the vacuoles, protozoans of fresh water would absorb too much water and would die.
