This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 沗 is uncertain. It visually combines 天 and 氺, but the original component relationship is unclear.
Since the meaning is unclear, focus on the shape: 天 (heaven) above 氺 (a water-like variant). Picture heaven pouring down water, but remember the meaning is uncertain.
For テン, use the top component 天 (テン) as a direct sound hook: 天 -> テン. Imagine the water below reflecting the sky above.