You let it pull you underwater, but then grab the ATV (it isn't actually a "heavy object" so you can just grab it) and float back up to the surface.
Thanks for replying! That's what I tried to do but my character wouldn't go up. Maybe I accidentally sat in the atv instead of grabbing it. It's hard to do because it happens so fast. Anyway, good to know that it's possible. I'll keep trying! :)
