I honestly do believe Ororo would usurp Mera's attempt at controlling the water within her body. Sure there's a scenario where Mera just pulls off a completely immediate, unexpected attack successfully, but 9/10 I think Ororo would sense what she's trying to do and be able to immediately counterattack with a stronger willpower and greater base ability. We already know Ororo can split water molecules, we already know she just punked HydroMan into humidity with a flash of her eyes. She can make animated water and cloud 'holograms' with her mind. She can sense across the other side of the earth and do all these things remotely. Mera is cool, I like her look, pure hydrokinesis is great on it's own, never mind the whole aquatic strength and durability and all that. I love her in the new Aquaman movie(I mean, Jason's there, so it's an automatic win), but Storm beat Wonder Woman, and Wonder Woman beats Mera, so.....