high water, flood tides
汛 centers on a sudden rise of water: high water or flood tides, the kind that surges and spills over.
汛 combines 氵 (water) with 卂, which likely contributes the sound and may suggest swift movement, giving the idea of rushing floodwater.
Water 氵 rushes in swiftly like a winged messenger 卂, bringing high water and flood tides.
For シン, imagine a sudden flood tide washing over a thin, fragile dam: thin -> シン, and the water breaks through.