the nautilus, the ray
鰩 centers on a specific sea creature: a ray (the flat fish) or a nautilus. These are two distinct animals, so the core idea is simply a marine animal of that kind.
鰩 combines 魚 (fish) with 䍃, which likely contributes the sound. The character was created to name specific sea creatures, but the exact historical development is uncertain.
The left side is 魚 (fish), and the right side 䍃 looks like a container or a net. Imagine a flat ray or a spiral nautilus caught in a net: 鰩 is that sea creature.
For ヨウ, picture a ray fish swimming in a yo-yo pattern up and down: yo -> ヨウ, and the ray glides through the water.