In the Downtown section, there is a specific letter tile that must be obtained by "shooting the bolts to cross the chasm". To get this, go to the construction site and to the area past the drills to where the water and barrels are located. Make
sure you are in the Mermaid Man costume. The area is just after you get past the drills, not the location where you delivered the Krabby Patties to the foreman. Instead, run to the right where there is another man. Look to the left and you will see a glowing red object on the wall. Use your water balls to hit it. It will drop a platform. Jump to this and repeat until you get the tile.