I’ve tried messing around fairly extensively with water in the latest release, so here’s a list of what I have and haven’t been able to do:
I have been able to convincingly link up multiple lakes and ponds on the same level, and do it without visibly diminishing the water level. The key to making this work, and hiding your work, is to use small steps and allow the water to fully equalise before opening up the next section of “river”.
I’ve also been able to run water underground from a large lake high in the mountains to a new pond on the next level down. To keep things clean, I started by digging the horizontal part of the “pipe”, and left blocks between it and the final pond location. I then dug the vertical shaft, and allowed the pipe to completely fill. I removed a single block at the bottom of the barrier, allowing the pond to fill slowly; and once the water level met up with the ground around the pond, I removed the rest of the barrier. This pond was full to the brim, but the game actually prevents rising water from overflowing its container. If you pour the water in from above then it will rise over the edge and cause a flood; but if you fill from the bottom up then your holding ponds won’t overflow (so that’s great if you’re not sure about the volume of water to be moved.)
There’s a really nice mod which adds the different terrain colours to the building palette, and I’ve used that to create some really nice level streams which look completely natural – they had the correct lightened grass borders and everything. So, if you’re just looking for a nice stream to connect two ponds, or to run to the edge of the map (the water stops and the edge and won’t “run off”), that’s quite possible.
However, getting water between the different levels is tricky if you’re going down, and virtually impossible if you want to go up. You theoretically could do it by getting all the water you want into a vertical pipe and then filling that pipe in from the bottom, forcing the water up… but you’ll have an issue with where the water originally came out of, since there’s always a fine layer of water that won’t flow away. Of course, you could build something over the top (and that means a block above the water “skin”/“puddle”); but you basically want to handle your water movement underground where possible unless you are planning to keep that stream/canal filled permanently.
Now, as for what’s not possible… pretty much everything else, unfortunately. Waterfalls are messy and impossible to control once they get going; and building a fake waterfall requires lucky placement of a lake both above and below your intended location. Oh, and just to make things more difficult, the technique for faking a waterfall only works when it’s on stone; if you try it on a dirt background it becomes immediately obvious. @micheal_handy76_mh has made some impressive fake waterfalls, but having tried to copy his technique, all I can say is that he has impressive dedication to his craft.
Running water over long distances requires a very large source, a lot of patience to make very small moves at a time, and often some trickery with filling in the bottom of the initial lake (again, do it in small sections or the water freaks out)… it can be done, but it’s the kind of thing you’ll want to test out and master in “throw away” worlds before trying it in a world you’ve built anything in that you’re fond of.
Of course, I’m hoping that the developers can spare some more time for water in the near future, and give us tools to effectively move it around (pumps, or even buckets LOL!) For the meantime, the best trick with water is to use a natural pond near where you want the water feature, and then modify it with small but “smooth” stages (i.e. once the water starts flowing, try to let it finish and equalise completely before doing anything else with it.)