support is given to the province, not the coast - if your unit could move to the province it self, it can support a hold in that province, or a move to any coast in that promise.
The only time a coast affects a fleets ability to give support, is if the supporting fleet is parked at a coast - a fleet in STP NC would not be able to move to GoB, and thus cannot support for instance Bal-GoB, but GoB can support hold STP NC.