Originally posted by Lethian:

my plan was to build a harbour so I wanted to dig out an area close to the shore and once done building it to fill it with water

Not really how it works. You can dig down deep enough to hit water (if within 8m) but you're not going to have a "break the dam" moment where you delete a wall and water comes rushing in. There either is water at a certain height or there isn't. It doesn't flow.