It shouldn't count steps. That would be silly. It should count swim strokes. According to the specs Garmin Venu counts swim strokes: https://buy.garmin.com/en-US/US/p/643260/pn/010-02173-11#specs
Are…
What is the point of counting steps in the first place? If you're counting feet hitting ground then put the sensor on the sole of shoes. 2 hours of mowing my lawn add around a 1000-2000 steps. That's only because I try to pull the mower with the non-watch wearing arm.
if steps are an indicator of health, then all movements should count, rowing, swimming, wheelchair rolls, all of these movements are better indicator of health.
Since Garmin markets these watches as activity trackers, then active lifestyles/health should be the focus, not the definition of a step.
Some people like to use the number of steps taken as a measurement for how healthy the are living. If that works for them then it is good. The important thing is to keep on moving your body.
I only use the step counter to get badges and do challenges. That is the only purpose I see for the steps. I'm way past using steps as a measurement for my health.