Seafarers of Catan fog scenario question
You should reveal the hex tile when you first build a ship that reaches it; you don't wait until you are building a settlement.

From the rules:


  When a ship (or a road) is connected to an unexplored
  area, a new tile will be discovered! If you place a ship or road
  that connects to the intersection of an unexplored hex, you
  must immediately draw the top tile from the pile and place it
  face up in the empty space.