Moray eel
鱓 centers on the moray eel, a specific type of marine eel. The meaning is narrow and refers directly to this animal.
鱓 combines 魚 (fish) with 單, which likely contributes a phonetic element. The character is used specifically for the moray eel.
The left side 魚 means fish, and the right side 單 looks like a long, twisting body. Picture a fish with a single long body like a moray eel.
For セン, imagine a moray eel swimming through a narrow underwater canyon: canyon -> セン, and the eel's long body winds through it.