For ages fishermen in Indonesia have build off shore rafts that fish could find shade and protection from sea birds under (also from hunters from down below). They get blown away by storms, then replaced again. The designs are still very primitive. You can see more videos here.
There’s two ways to view them, one is that they help deplete the oceans of fish, by making them easy to catch. The fishermen still need to go out 30 miles or so, this is expensive (because they use fuel not solar electric or sailing boats). Indonesia is actually stimulating the creation of Rumpons, here 1000 of them, but its not clear if this is good for fish stocks.
Because the ocean is warming due to more CO2 in our atmosphere, and this in turn means the air is not cooled causing highr temperatures, there is a need to increase the Albedo of the oceans. Of course they are vast, its not easy to make any change to the global climate, and this is a good thing. It seems that simply blocking the sun from getting into the ocean is a good way to keep it a bit cooler. It does radiate a lot, and normally it absorbs nearly all solar heat that hits it.
Sealife can really thrive if it gas somehting to hold on to. Coastal sealife is much more diverse both because of the nutrient rich water usually upwelling from the deeper ocean and because of the many nooks and crannies life can cling on to and hid in. The open ocean doesn’t have these things which is why it can really be quite dead. We can change that.