ocean, swamp
瀛 centers on a large body of water: an ocean or a swamp. Both meanings point to a vast, water-filled expanse.
瀛 combines the water radical 氵 with the phonetic component 嬴, which likely contributed the sound while 氵 indicates a body of water.
The water radical 氵 on the left signals a body of water. The right side 嬴 looks like a tangled swamp or a vast ocean with its many strokes, reinforcing the idea of a large, watery place.
For エイ, imagine an 'A' grade ocean: the letter 'A' sounds like エイ, and this kanji represents the vast, top-quality ocean.
the pacific side of Japan; eastern sea