The quick details.
Side: Dawnguard
Quests Active: Chasing Echoes, Disclosure
Problem:

I have gone through the dialogue where the hero and Serana realize her mother is back at the castle. She tells me about the jetty. I fast travel to the boat, take the boat over. We walk the path to the /left/ to find the jetty, she tells me as we approach "its just around this bend" great, we clear the skells out. Awesome. I approach the door and nothing. Door requires a key. Ok great. I talk to Serana. Her only dialogue option is "What is this place" where she explains what the dock was used for and that we need to use caution when we enter. Then...nothing. No other dialogue other than the regular companion dialogue and asking her about her vampire past is available.

She wont open the door. We just stand there like tards. What am I missing? The journal says to speak to Serana. I did that, she has no dialogue options available. I went back to Fort Danwguard, asked her again, again no dialogue. I tried running the whole darn way from the Fort to the Castle, ON FOOT, no dialogue options. Nothing.

I can understand, that this was rushed DLC but, I paid money for a finished product. Not one that is released at full price unfinished, to be patched in later. Ridiculous.

I never had that bug, the door didn't require a key....

There are no other dialogue options at that point, not till you go in the door. Did you have the quest selected in the journal? Did you kill everything? You could try changing the quest marker and doing something else and then come back to it, maybe it will update the quest then. There is no key to the door, Serana is not going to say anything else, you just need her to go in the door.