The phrase "float on your back" is grammatically correct and can be used in written English.
Example: "After a long day of swimming, I like to relax and float on my back, gazing at the clouds above."
Jump approximately eight feet from volcanic rock into the deepest one, or just float on your back in the warm salt water, marveling at the blue sky, sun, and birdsong.
There cannot be a better way to swim or float on your back than this.
("Spread your arms and legs far apart and try to float on your back").
You float on your back in the dark in 10 inches of water saturated with Epsom salts.
There is no one but the sea birds and the perfume of lavender and thyme, and you float on your back, suspended in eternity.
"If you're having a moment," she said, "stop, float on your back, have a little sing and think of Beryl Burton who beat all the men in a cycling race by just keeping on going".
Float flat on your back, and do a flutter kick with your legs.
Float on your backs, Mr. Boscarino said, feet first.
"That's what you see when you're floating on your back," Olivia told us, rubbing the gray crayon down to its nub.
You were in the middle of the great downturn, which meant that you spent most of your time alone, floating on your back in the ocean or getting drunk at the bar or walking the beach in the early morning before anybody was up.
And she knows that if you're out boating, and the boat sinks, you can make it to shore by resting every so often by floating on your back: "Taking deep breaths will raise your body to the surface like a balloon.
