Whenever I place a water square - big or small - it just moves the coastline around a bit.
I had my bots place a large square of what was to be a small lake, but ended up as a checkered piece of land with small water filled holes. While they're of course free (except for a factory and some power), it takes A LOT of them to dig a channel, as some of the previously freed area suddenly becomes a small strip of land again...

But great idea, though. I intend to use it to make a moat around my "castle" :-)

same problem, looking forward to an update

This is a fundamental problem with how the game tiles land/water. It's not specific to the mod at all and is why the base game doesn't let you blueprint landfills. It sounds like 0.15 will bring in some changes that fix it though.

Still there in 0.15