Okay this is not that big of an idea but I did not see or read anything about it from anyone.
I stumbled over this because I wanted to have a lake in my settlement but I did not want to “destroy” the landscape.
Soo what toi do?
Creating a lake with blue slabs?
Nooo, the water physics are great, why not use them.
First you should dig out at least two layers, you can dig more or less …
And then dig a tunnel from the next lake, most big lakes are two layers deep, to your artificial lake.
After the tunnel or undergound path is clear, water will flow through it
and fill up your lake
Here is a screenshot out of my current game:
Since there can be not more water than there was at the start of the map, the water level will get a little bit lower on the original lake.
If you tunnel more lakes together the water level will be a little bit higher on your lake.
I hope you understand what I mean
And yes, it is not that great of an idea and maybe most of you knew this or even used it
But maybe there are a few out there like me, who are not so fast at thinking 
hrr hrr
This is for us(you).
cya