whirlpool, undercurrent
洑 centers on water moving in a hidden or circular way: a whirlpool or an undercurrent. Both meanings describe water that is not flowing straight on the surface.
洑 combines the water radical 氵 with 伏, which likely contributes the sound フク and may suggest the idea of something hidden or submerged, fitting the undercurrent and whirlpool meanings.
Water 氵 hides a person 亻 and a dog 犬 on the right, like a whirlpool pulling them under into an undercurrent.
For フク, imagine a whirlpool making a deep 'fwoosh' sound as it spins: fwoosh -> フク, pulling everything into the hidden current.
—