Answer
Protists in hypertonic habitats would need to have a way to hold onto water, which would otherwise continually diffuse out of the cells. One way to do so would be to have cell membranes which were more permeable to water when it moved into them via water channels or membranes which were more resistant to the movement of water, limiting osmosis out of the cell. They could also accumulate solutes to help hold water in their cells. Protists in habitats of variable salt concentration would probably be able to vary their internal solute concentrations to compensate for external changes in solute concentrations.
Work Step by Step
They key here is to see that cells can either resist changes by slowing the mechanism of those down (as with some described in the question) or compensate by altering their internal solute concentrations to match external conditions, thus avoiding osmotic water loss or excess water uptake.
