I've search numerous forums and tried their suggestions. Alas, no key to the undercfoft.

Can you provide a bit more information?

Do you have any mods installed?

Do you need a key?

Thanks for the replys. I've been away for a bit.

Details:
Win7 64bit, SP1, 4GB, NVIDIA GTX275, lots of HD

Load sequence: Skyrim.esm, Update.esm, Dawnguard.esm, all others

Game play: Get the quest; either leave Serana at fort or take to castle, get elder scroll from Winterhold or defer, go to castle, get sunken treasure or defer, kill skeletons, stopped at door to undercroft by lack of key, return to Serana at dock, go through dialog, return to door, still no key.

Mods: Yes, a number of them.

No, you do not need a key. When you meet the Dawnguard there for the final battle, it will not need a key.

Serena is the one who leads you into the undercroft, that is why there is no key. So, if you are at the point that I think you are at, why are you leaving her at the dock when she is the one who gets you into the back door of the castle?

I am pretty sure this is correct, you need her with you to complete the quest so just take he and you should be fine

The above two post are what you are missing.