Of course, their protection is somewhat futile as they can hardly come out from a fight with a dragon unscathed, and yet what they can do they do to protect themselves:
Still they had a little time. Every vessel in the town was filled with water, every warrior was armed, every arrow and dart was ready, and the bridge to the land was thrown down and destroyed, before the roar of Smaug's terrible approach grew loud, and the lake rippled red as fire beneath the awful beating of his wings.
Of course, by cutting the bridge loose we are told that this makes the situation worse for Smaug. The lake is "too deep and too cool" for Smaug's liking, and he recognises that the like is "mightier" than him. However, he is still able to set fire to the wooden island until Bard successfully shoots him in his unprotected part of his belly and he dies.