This question was raised in an XKCD What-If post (in footnote 3):
When a swimmer is submerged and moving at top speed, the drag from the water is equal to the thrust they generate by kicking and ...
...whatever whatever the gerund form of the verb is for the things your arms do while swimming. My first thought was "stroking", but it's definitely not that.
What is the verb to use to describe what the swimmer's arms do?