As good as they may look on land, in water they just seem a bit out of place having so sharp edges, and also being totally rigid. Some examples below:



My suggestion is to at least apply some blur around the edges of underwater shadows, ideally make their location oscillate a bit around some point, to create an illusion of changing refraction effects due to waves.
To go a step further IMO all shadows cast by units floating over ground should be blurred as well. only point-like sources of light can cast perfectly sharp shadows and the higher is the altitude of an object the more blur should be observed.
It's just a quick suggestion for your consideration. I personally have no idea how much effort would it take to implement something like that in spring and how much the performance would suffer because of that. Yet even a simple blur would make it look way better so maybe its worth considering :)