What I don't understand is how the top reservoir is floating when filled with brine. Are the small floaters enough to hold it up?

Otherwise I love the fact that's simple. Simplicity scales. It's also salt water, so assuming they're not putting anything else than NaCl, it can break and it's no big deal

Have some air at the top of the top reservoir, assuming that the top reservoir is at most about 10 m deep in water (to avoid damage from storms). Or have the air in separate chambers fixed to the top reservoir.