> Most land animals have a skeleton not just to provide physical scaffolding to hang tissue on but because we need a store of calcium to mediate blood pH, something sea life doesn't require thanks to those minerals in the water. That's why you can have sharks which are mostly supported by cartilage

Well, the other reason sea life doesn't require rigid bones is that water provides a lot more buoyancy than air. Jellyfish work fine in the water; on land they're immobile puddles of glop.