Hydregion can't evolve cause Hydregion is the last stage.
This confirms a few things. First being that Ash beats Stephan and that Kotetsu beats Bianca(yeah sorry everyone that thought she would win). So this leaves Kotetsu and Virgil as known competitors?
As for his impending battle against Kotetsu Hydregion will indeed be a pain, but I think they will they will have it vs. Ash Krookodile which if you remember gained Dragon Claw when it fought Iris Dragonite upon evolving so I think that will be a good card to have.
Personally if Ash does lose I'd prefer it too Kotetsu instead of Virgil since I don't care for him too lose to a team of Eeveelutions...