Do not exhaust yourself; keep your hands quiet and tread water.
His boots, too, were acting as a drag, yet there was no time to tread water and kick them off.
At first it was easy to tread water: the salt brine upheld him.
Some one says: "To tread water is like running up-stairs rapidly."
He began to tread water, lifting his mask, then rinsing it because it had fogged a little.