stream which leaves the main branch then later returns
汜 refers to a specific type of stream: one that branches off from the main flow and later rejoins it. The meaning is narrow and technical, describing a geographical feature rather than a general concept.
汜 combines the water radical 氵 with 巳, which likely contributes the sound. The character is rare and its exact historical development is uncertain.
The water radical 氵 suggests a stream, and 巳 looks like a winding path that leaves and returns. Picture a small river branching off and then curving back to rejoin the main flow.
For シ, imagine the stream as a 'she' who leaves but always returns: she -> シ, and the water always finds its way back.