This week our club swim coach planned a workout that includes a 30 second vertical kick drill. So you're just kicking in place without moving across the pool. Unfortunately Garmin Connect workouts don't seem to support that properly. If I create a workout step with Step Type = "Swim" it doesn't allow me to set the Distance field to 0; it gives this error: "This duration must be greater than or equal to 25".
For now I have set Set Type = "Rest" even though it really isn't a rest step and set Duration to 0:30. Garmin really ought to have a better way to handle such drills for pool swim workouts.