Dive HM came in around the.... 3rd Gen games, I think.
Anyways, Diving has the unique ability to explore things underneath the surface of the water.
How does this apply to the current region?
Well, we currently have quite a few places which have water. Water+ Dive HM= New content.
Therefore, once Dive gets implemented, there is a chance that the waters of Kanto will get a face lift.
For example, do you remember the small pond in Viridian City?