There's a theory floating around about why the Night King keeps Jon Snow and Co. on that little rock all night — he knows that the dragons are coming because he has visions similar to the ones Bran does and is therefore using the group as bait so he can kill and turn a dragon for his White Walker army. Now, while that's cool and all, it's not so much as hinted about in the show. We don't know, with any certainty, if that's why he chooses to hold his army at bay, so it's unfair for us to assume that's the reason.
Going off of only what we see in the episode, it doesn't really check out that the wights would wait for the lake to completely freeze over to attack them. With all the thousands of wights at the Night King's disposal, why didn't he have them strut across the bottom of the lake, Pirates of the Caribbean-style, and stack up on one another until they can pop out and attack the humans? It seems like a better option than waiting hours upon hours for the deep lake to freeze over.
