I had one settlement asking for lots of water, so thought I'd install a large water purifier to cater to this request.
Only then did I discover it can't actually be constructed!
It says I must place it in water and it won't allow it to be built anywhere else.
I then tried to build one at Nordhaven, which is close to an oceanic bay.
But as the water was outside the construction limit, I was disappointed/frustrated another time.
I eventually found a working one in Diamond City, immersed in a lake.
If that is what is required to make one, then it serves no purpose in the game!
All the settlements I've visited have no deep water bodies in their construction zones.