drown, be immersed
湎 centers on being submerged in liquid, especially to the point of losing oneself: drowning or being deeply immersed.
湎 combines 氵 (water) with 面, which likely contributes the sound and possibly the idea of the face being covered or submerged. The exact historical development is uncertain.
Water 氵 covers the face 面, like someone drowning or fully immersed in liquid.
For メン, imagine a man drowning: man -> メン, and his face is submerged in the water.