Como é que se diz isto em Inglês (EUA)? the action of swimming with fins on your feet’s !

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@ParkT
Hey, you teach scuba diving. That's pretty cool.

Yeah, technically, the word "finning" is in the dicitionary, but nobody uses it. Almost never. I didn't even know it was a word except I just made it up in my mind and was curious if it was in the dictionary or not, so I searched for it.

https://www.google.com/search?source=hp&ei=...

I think if you use it in a sentence, most people will understand what you mean from the context though.

Example sentences:
He frantically finned himself to the surface.
She was finning so slowly I thought she was going to drown.

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By the way, your sentence should be written like this:
the action of swimming with fins on your (FEET)
No " 's " on the word "feet".