large boat
舸 centers on a large boat or vessel, emphasizing size and capacity.
舸 combines 舟 (boat) with 可, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a type of boat.
A boat 舟 that is 可 (possible, acceptable) to carry many people: a large boat.
For カ, imagine a large boat that can carry a car: car -> カ, and the boat is big enough to hold it.