Even in a storm, just a few meters below the surface (half the wavelength), the sea will be calm.

The bigger issue with this idea is that it's a megastructure sitting in the ocean, and salt water turns everything it touches into shit. Oh, and there's very little energy storage potential from just a salt gradient. You need to move way more water, to get less energy, but your container costs are fixed.

Land-based pumped hydro has no shortage of engineering problems (and risks if, you know, you get a dam collapse), but this has colossal capex costs.