If you are curious about recent game developments, this company is working on water simulations in voxel-based worlds with gravity, buoyancy, rivers, waterfalls, surface tension, pressure… They just had a Stream about it today.
https://www.keenswh.com/vrage/
vrage3 is not a game, but a real-time physics simulation engine. (They have released games before, so they will definitely be considering using the new engine for a game in the future.)
Currently they don’t care about whether the water looks shiny/realistic, instead they want the real-time behaviour to be realistic. On a planetary scale… The current Proof of Concept is 2D with some first tests in 3D. In the stream they showed how they could change the direction of gravity: e.g. a lake fell upwards from the planet and pooled in space around the gravity source. And (in 2D) they can pour water (rain) on the planet and it follows gravity and flows into oceans, or into caves, or it forms a blob in the hollowed out core of a planet.
The demo is far from having all features, but it’s daring that a company is starting to tackle this task, instead of saying - “obviously it’s impossible and performance will be bad“. With flowing water it’s even more difficult to synchronise in multiplayer, because there are many more moving parts that could go out of sync. The demo is singleplayer but their other physics games are multiplayer for 32 players, so they know what it takes.
Also none of their games are procedural like No Man’s Sky. Their planets (and riverbeds and lakes) are predefined but deformable. Their approach doesn’t deal with NMS’s problem of having to procedurally generate a planet with plausible river beds. That’s probably still unsolved? But we’ve gotta start somewhere. 
