fast boat
艎 centers on a fast boat, a swift vessel used for quick travel across water.
艎 combines 舟 (boat) with 皇, which likely contributes the sound and possibly the idea of something grand or swift. The exact historical development is uncertain.
The left side 舟 is a boat, and the right side 皇 suggests an emperor. Picture an emperor's fast boat speeding across the water.
For コウ, imagine a fast boat named the "Kou" cutting through waves: Kou -> コウ, and the boat is swift.