iOS 6 Beta is not a showcase. Devs are under NDA. It's a test, to find and squash bugs. Again, if you're a developer, you understand the value of being feature complete and working with a copy or your production data during the last Q&A phase before mass deployment : limited datasets used during active development might not be able to trigger all conditions that might result in unwanted behavior.
Thus there is no reason to believe this isn't actually Apple's live map data or at least a complete copy of it. Otherwise, the beta has no real value.
You mostly support the debate against yourself here. The beta is for Devs to test their apps and make use of the new APIs, not to showcase the new iOS, as you rightly say. Now how does an old dataset with wrongly labeled oceans affect apps that devs are testing, like Facebook or what not? It doesn't, therefore the beta does not necessarily need to have the latest dataset...or at least apple obviously has reason to think so. You still haven't given a reasonable counter to my point that apple have many more cities in 3D from C3 that's currently not enabled, so again, we do know that apple is holding stuff back from the beta.