large reptile, water lizard
鼉 centers on a large aquatic reptile, specifically a water lizard or similar creature. The given meanings point to a single concept: a big, lizard-like animal that lives in water.
鼉 is a phono-semantic compound: the bottom component 黽 (a type of frog or reptile) suggests the meaning, while the top part likely contributed the sound. The exact historical development of the top component is uncertain.
The bottom part 黽 looks like a creature with a big head and legs, and the dense top adds to the image of a large, scaly body. Picture a giant water lizard emerging from a swamp, its shape filling the whole character.
For タ, imagine a large water lizard basking on a tar pit: tar -> タ, and the sticky black tar matches the dark, ancient feel of this rare kanji.