The “mining while mounted” feature that you can unlock in The War Within does not affect mining while mounted on a Sea Turtle in water.
If you are on a sea turtle, in the water, and mine …you’re dismounted as though you didn’t have the feature unlocked on your character.
Probably because you can’t land while underwater, and gathering while mounted has always required you to be firmly grounded.
That’s perhaps the reason, but still a bug that should be fixed, since you can gather while not grounded in the water.
Adding that it will dismount you from any mount when mining (I assume herbing as well) while underwater, even with the mining while mounted profession knowledge in TWW. Very disappointing, so much so that I generally avoid underwater nodes, unless a portal takes me to one, lol.