I had a fanciful thought today. I've started reading about the caisson breakwaters used for the Normandy invasion (Mulberry Harbors for any who are interested).
Up to this point, I've had little inspiration in the realm of floating breakwaters. To be effective they have to be massive, to be massive enough, they are beyond the financial reach of anything much less than a city-state like Monaco or Dubai.
On another thread I was looking at freshwater storage/ballast.buoyancy issues, and placement of such to avoid loading issues when water is being used too quickly to replenish.
I had the thought: what if the mass of a floating breakwater were provided by freshwater, while the rigidity were provided by concrete. Fresh water is about 3.5% less dense than seawater (varies a bit with location and temperature, but call it 25-35 Kg difference per cubic meter.) If you built a big enough concrete enclosure filled with freshwater, you could provide enough buoyancy to float the weight of the concrete, while still having a large mass. and a resource that is serving a secondary purpose.
I know this has been considered using bags of freshwater on the surface, but two problems with that: first, the flexible polymer bags tend to degrade in sunlight rather quickly. Second a bag of water floating in water is not de-coupled from the wave action, it just transmits the wave to the other side. A rigid structure decouples the mass inside, and to a degree, the mass of water on the leeward side, from the wave form.
You still have to have a pretty massive structure to benefit from the minimal buoyancy of the fresh water in sea water. You still need a fairly large structure to have a size/depth/wave height ratio that effectively de-couples from the larger wave form. The engineering for making such a large, relatively flimsy structure rigid and durable may not work. But it's an idea I hadn't seen explored before.