"Water under the earth" (Psalm 136:6, Exodus 20:4) = the ocean. Exodus 20:4 prohibits idols of things in heaven, on earth, or in the water under the earth — if this isn't the ocean, then the idol prohibition doesn't cover fish/sea creatures. Psalm 24:2: "founded upon the seas, established upon the rivers" — earth (dry land) is simply above water level.

Exodus 20:4 must include sea creatures in the idol prohibition (Dagon was a fish god). "Water under the earth" = the ocean. Psalm 24:2 confirms: earth founded on seas and rivers. "Earth" means dry habitable land above sea level. Rivers flow under your feet as you walk — that's earth upon/above water. Not secret primordial waters.