The "sea of clouds" is best observed just before and just after sunrise at the summit of Huangshan (especially in the area of "Refreshing Terrace" and "Lion Peak"). One needs to spend the night at the summit—or preferably several nights, as the weather is changeable and the "sea of clouds" doesn't always appear. It is, however, well worth both the time and the effort. This is one of the most spectacular natural phenomena that one is likely ever to experience.
I was here in late May 2012 and spent five days in the area, walking up the mountain (approximately 4 hours) as well as most of the way down. Cable cars are available, but you miss some remarkable scenery with that sort of transport. Some days are quite misty, but this, too, presents beautiful imagery. Do make certain to take the hike around the West Sea canyon and traverse the White Cloud scenic area up to charming Fairy Tale Bridge.
Once you've descended the mountain, spend some additional time (3-4 days) in the area of southeastern Anhui province and visit Unesco-cited Huizhou villages like Zidi, Hongcun, and my own favorite, Tangmo, which is not as touristy but absolutely delightful. Note that most visitors to this part of China are themselves Chinese; Westerners are regarded (and welcomed) as curiosities here.