Water sounds kike it will be really tricky. I think there should be infinite source blocks, but they should not be abailable to players in a normal game. They should only be created during the RWG process or in creative mode.
It would probably be relatively easy to bake rivers into Navezgane. But getting rivers functioning in RWG wlild probably be a nightmare and would add to the RWG generation time.
I think lakes should count as infinite sources. But in terms of water fill, there should be a limit. Ideally water would flow until it reaches sea level, and sea level would be fixed so that it can never rise. I.e. water that makes it to sea level would automatically cancel out at that level.
But yeah, there are probably server crashing possibilities out there. Perhaps there could be a server option to limit or turn off flow for larger servers. Internal testing would probably yield interesting results.
However, good water has been done... I believe several years ago it was Roland who posted a video of a game where water would drain and fill other areas if the player created a situation where it could happen.
Like spice, the water must flow.