They have an alternate attack when swimming. Instead of rolling in the water they bite. The bite can be used to gather stone and if you're in a body of water with a lot of rocks you can get an excessive amount of stone faster than on land because you don't have to sit through the standard animation.
Equip your Doedicurus with a saddle, pick it up with an Argentavis and fly near rocks as it automatically harvests the stone as you fly. Level up the melee damage to harvest more stone with each swing. Level up your Argentavis’ weight capacity be an insane stone harvest team.