I wasn't able to drink from Ebonmere to choose a moon sign. I think it was either because I wasn't wearing all the Nightingale apparel or hadn't done enough radiant T.G. quests. I went back to a save before entering the Twilight Sep..So will putting on all the Nightingale gear or doing more T.G. Radiant quests allow me to complete the final T.G Quest?

You choose a sign by standing on it's glyph on the floor. You should be able to choose a new one once a day. The frustration is that when you travel to the Sepulcher, you'll usually step on one on accident, meaning you then need to wait 24hrs to get the one you came for. You shouldn't need to be wearing the gear to do this and once the Nightingale questline is done, there's pretty much zero integration with the Thieves Guild.

But Nocturna said I had to drink the water from Ebormere , there is an quest arrow in the pool of water but I can't find a way to drink from it.

You don't actually drink anything if I remember correctly. After Nocturnal vanishes, there is supposed to be a conversation with Karliah in which you can ask her about the different Nightingale powers. Then you can stand on one of the floor glyphs to choose your power. Best of luck!

I tried but all it said was "This person is busy"

Where is your quest marker? If it is still on the watery stuff where Nocturnal was, try waiting a few hours to see if your quest updates. Also, try looking here to see if there are any bugs like the one you are having: http://www.uesp.net/wiki/Skyrim:Darkness_Returns

Thanks, will do it over and wait if necessary

Sure thing! I hope you are able to fix it.