>>42606497I no longer stand by the wetsuit thing, but Walrein's hair is stylized to look like splashing and his tail is I think stylized to look like a clam.
I think on reflection Tentacruel's red nodes do look a bit like they're full of poison or water, but the psychic typing would have been consistent with other Psychic Pokemon like Slowking (red jewel), which is related to Tentacool in a roundabout way, and its general alien mind-controller look.
Cloyster isn't just hard, it's clearly protected by a secondary shell of metal. It's also conceptually based on a theme of protection and hardness -- steel type's basic essence. The dark typing goes beyond the colour to its cowardly and devious strategies, as conveyed by a malicious expression. Poison type is described by the corrupted look of the shell and, yes, its purple colouring that suggests an ability to deliver poison through the spines. Its expression also conveys poison typing.
Horsea and its evolutions have the furled tail of seahorses, so I couldn't argue that the furled tail was an invention to convey water typing with a whirlpool symbol; nonetheless, the furled shape is visible on many water-related Pokemon that were invented: Lapras, Slowpoke, Squirtle, Alolan Raichu. It's also on a many non-water Pokemon like Meowth, Rattata, Vulpix, and Jigglypuff; so, the shape conveys different traits in different circumstances. I think it's acceptable that its use often deepened a link with the idea of water so that that type was legible. In the case of Kingdra the seaweed/coral shapes are both a rendering of the animal inspiration (to establish the pun underlying its dragon type) and a furtherance of its connection to water type.
Omanyte I really can't defend. I think the spiral shape is conveniently symbolic of whirlpools, but there is really no other way to draw an Ammonite. In contrast, I think Poliwag's spiral, a convenient actual phenomenon, was appropriated deliberately to demonstrate water typing.
