First, you want to avoid reaching land. What I mean by this is stay on floating pieces of wood and staying away from the ground by the columns and chunks of concrete by the walls. You'll still wake a few zombies, but you shouldn't have to use more than 6 shotgun shells (I've gotten away with using only 4 once!) Keep doing this until you reach the pipes where you can unlock the door.

I recommend having the gravity gun and shotgun handy (quick switch between the two). Gravity gun to move pieces of wood around and grab health and shotgun to kill zombies.