swim
泅 centers on moving through water by swimming. It is a specific character for the act of swimming.
泅 combines 氵 (water) with 囚, which likely contributes the sound. The exact historical development is uncertain, but the water radical clearly relates to swimming.
Water 氵 on the left and a person 人 enclosed 囗 on the right: imagine a person trapped in a pool of water, forced to swim to stay afloat.
For シュウ, picture a swimmer doing a powerful shoo! motion through the water: shoo -> シュウ, as they glide forward.