For the first few passes caustics are always are very dark. You need to wait for more passes when caustics solution gets more refined, they should get significantly brighter. Another thing is that my water material is unrealistically darkened and tuned for rendering without caustics. If you want to use caustics, you need to click on colour swatch bext to refraction and set its colour to pure white. And finally make sure that you've enabled caustics in water material, as well as in render setup>performance tab. You can uncheck "generate caustics from enviroment" if you want faster rendering, or leave it checked on if you want more accurate result.
P.S. at the moment caustics doesn't work well with region render, so unfortunatelly you need to render full image to get accurate results.
P.P.S. box is just a light blocker to prevent light leakage through one sided floor mesh. It's has no significant importance in the scene.
