Water color capability:
With mypaint brush library, unless it becomes a focus, I don’t expect to see water painting abilities similar to what Krita can do in the near future. Custom brush dab support has been requested before and would be able to make it much easier to simulate water paint like effects. The lack of texture really is what sets mypaint back the most currently imo. so maybe at some point I could look into it. There are however other things that have higher priority than making image dab support for libmypaint 
even though it would be very useful and expand the room for brush improvements a lot.
Brush import:
You can indeed import other brush sets manually and they may or may not work entirely as expected, which it also sounds like you’ve experienced. I’m not sure why though, maybe because they have been created with a different library version… or there could be a bug something that i’m not aware of… 
Be aware that the newest mypaint library uses another brush format… so some brushpacks will currently not work because we use an older version. Although you could probably convert them manually if you really wanted to try out those brushes.