float
沲 centers on the idea of floating on water, as an object resting or moving on the surface.
沲 combines 氵 (water) with 㐌, which likely contributes the sound. The exact historical development of 㐌 is uncertain.
The left side is water 氵, and the right side 㐌 looks like a person or object resting on a surface. Picture something floating on water, supported by the surface tension.
For タ, imagine a floating tar barrel on the water: tar -> タ, and the barrel floats on the surface.