I think the lack of raw materials makes it very hard to make simple things to then evolve into more complex things, building tools with wood, then stones, then metal. I'm not sure that young could smelt metal, etc, in the ocean.
I expect oceans have plenty of raw materials. Meanwhile, many things are easier in the ocean. For instance, buoyancy counteracts gravity, so it's much easier to lift heavy objects. And it's easier to transport them long distances. If you need air and a dry environment, all you ever have to do is go up (and maybe set up a platform).
