ferry, fog, flowing
沆 centers on water in a broad, misty, or moving state: a ferry crossing water, fog over water, or water flowing. The meanings connect through the image of water spreading out or moving across a surface.
沆 combines 氵 (water) with 亢, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with broad water or mist.
Water 氵 beside 亢: picture a broad river with a ferry moving through misty, flowing water. The right side 亢 looks like a high platform or pier, from which you board the ferry into the foggy flow.
For コウ, imagine a ferry captain calling out 'Go!' (コウ) as the boat pushes off into the foggy river. The sound 'Go' cues コウ while the ferry moves through the water.