uhhh Rotom-c is a great way to keep most Water-types in check, it resists their STAB and poses an immediate threat with volt switch and leaf storm plus, Rotom-c is able to use volt switch as they switch out and give you an immediate advantage unless they happen to send in an electric immunity, most of which fear Leaf Storm. most of the other grass types available to really well against the waters too such as Sceptile, Lilligant, Tangrowth, Amoonguss, Roselia and even Sawsbuck! Poliwrath's a good way to handle the water-types too, especially Kabutops and Crawdaunt.
Speaking of Poliwrath, we'll probably need at least two ways to get rid of him for Aggron to do his best. Poliwrath is a major roadblock to Aggron and he's going to have a lot of trouble sweeping unless we can take him out first.