Come on in, the water's fine:

To help keep your Sea-Monkeys water at a cozy 70+ degrees, pick up an aquarium thermometer from your local pet store (Wal-Mart...). These inexpensive units are a thin, smallish piece of plastic that will stick right on the OUTSIDE of your Ocean-Zoo tank and help you chart the comfort-zone within.

Check out this experiment on how water temperature affects Sea-Monkeys®.