So I believe the current system for water is height-map based. Can you possibly map a height-map onto a sphere and simulate a water planet? Use radial coordinates(angle A, angle B, dist from center) instead of XYZ? I guess there would be distortion near polar regions but you get the idea.
While this is technically possible, I don't think it would be applicable widely enough to be worth investing a large amount of time in. If someone approaches me with a dire need for such a feature, maybe
Though again technically possible (though it would be a few times slower than the fluid currently present, and would not include waves like a heightmap), I'd need someone to express a need for such a feature.