This PR should let you do what you want:
It adds a new compositeMode property. When set to true, the alpha channel is set to the calculated thickness if the background color has a zero alpha component at that pixel. So, if you clear the scene with a color like (1,1,1,0), you will see something like (https://playground.babylonjs.com/#KXUQLC#1):
Note that the rock background comes from a div element behind the canvas, it is not part of the Babylon scene.
Also, there won’t be any refraction effect under the fluid object, because the HTML background is not accessible to the fluid rendering code. Instead, a fixed color is used for the refraction, which is the color (rgb components) you set as the scene clear color.