I'm stumped. I have a map of watersheds of the United States: complex polygons tracing across the US that all meet the ocean eventually. I want to buffer these polygons a few km into the ocean. A Buffer operation will buffer each polygon individually, creating overlap. I can't have this. What I'd like is a sort of Thiessen bubble effect only where no polygons exist (the ocean). The Create Thiessen Polgons only accepts point inputs. There are hundreds of watersheds so I can't do this manually. What can I do?
I'm using ArcInfo license on ArcMap 9.3.1 SP2.
Attached is a visual of what I'm looking for (made in Photoshop).