Originally Posted by
Vertigo
It was a trap. When Bran has his vision of the army, they're in the exact same spot - you can see the 'island' if you look closely. They didn't move on at all after that point. The Night King has some sort of ESP similar to Bran, so the whole thing seems calculated for him to get a dragon.
So why he never went spearfishing over the days of waiting - one, live bait is more useful than dead. Two, it looks like they only had three of those javelins; the other Walkers are carrying the ice swords we normally see them with. They were saving them for the dragons.