Wrist HR accuracy for swimming?

I have generally accepted that Garmin wrist HR monitors are not accurate at all during an aerobic workout. Yesterday I did a hard Zwift ride, and my HR strap was measuring 160 bpm while my Forerunner 170 said 86. This is pretty normal for cycling workouts so I just don’t use the wrist monitor for those. However, this morning during a swim session, I looked at my watch and it said 150 after a sprint, so I started counting my pulse manually. I found it tracked surprisingly close to the watch readings every time. Is there a reason the wrist measurement would be more accurate during swimming than cycling or running workouts? I have paid little attention to the HR data during swims so far, because I just assumed it was wrong and that the water probably makes it even less accurate.