I think you’re right on the water - if your current water is a separate material than the landscape (I’m assuming it has to be otherwise you’re working some crazy voodoo) then you’re probably better off focusing less on the water and more on the landscape itself. If that’s the case, ignore my thoughts on the waves.
The picture you linked shows vertex colors on landscape layers - exactly how many landscape layers do you have in this island material (or will have once its done)? Are they all using unique textures? The reason I’m asking is because if I have a jungle environment that sits above the beach level and leads into a volcano or mountain, I’d want each of those biomes to have their own fundamental set of textures (and not just color changes). I think this is what you’re doing, but I’m very surprised you’re able to get that many textures onto a landscape without it going black due to texture input overload.