fish, still
淰 is a rare kanji with two seemingly unrelated meanings: fish and stillness. The connection is unclear, and it may represent a specific type of fish or a state of water being still enough for fish to be visible. Treat its standalone meaning as uncertain.
淰 combines 氵 (water) with 念, which may contribute a phonetic element. The exact historical development is uncertain, and the connection between the water radical and the meanings 'fish' and 'still' is not well documented.
The water radical 氵 suggests a connection to water. Picture a still pond where fish are visible beneath the surface: the water is still, and the fish are present. This ties the two meanings together.
For デン, imagine a den of fish resting in a still underwater cave: den -> デン, and the fish are motionless in the water.