This skull is located on the pipe that is against the wall on the far side of the map, above the water. See the linked YouTube video below.
Note: Float the monitor at a greater height, and press Up on the D-Pad to get back into Spartan form. As soon as you do that, start holding the button to pick up the skull (Default: X), so that you get it as you start falling.
The Longshore skull starts at 0:26.