boat, ship
舡 centers on watercraft: a boat or ship. The two given meanings are essentially the same concept at different scales.
舡 combines 舟 (boat) with 工, which likely acts as a phonetic component. The exact historical development is uncertain, but the boat radical clearly anchors the meaning.
A boat 舟 with a simple, sturdy 工 shape on the right, like a work boat built for practical tasks.
For コウ, imagine a boat sailing into a cove: cove -> コウ, and the boat 舡 glides into the sheltered water.