As with any exercise, it depends on intensity and duration.
I believe resistance training means anaerobic exercise, i.e. short and intense, and you stop when your muscles cannot do it anymore.
Cardio means aerobic exercise, long and moderate, where the limiting factor is running out of breath.
Swimming many laps sounds like cardio to me.
But if you do one lap at maximum speed, and then take a break, it will be more like resistance.
You gotta be careful with swimming. It takes a lot of energy and can wear you out fast, but you do not “feel” it , as it puts minimal strain on muscles or joints, you do not overheat or sweat (unless you are really into it).
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