3D prints are too imperfect, i tried a 3D printed block for a raspberry pi before, and while it didnt leak, it wasnt quite what i'd call "comforting"

3D prints are very hard to get watertight once you start doing bigger things

If you print at 100 microns it should be fine. Also maybe a little bit of "reflow" at around 120c for a minute or so to melt the layers together. 

overextruding really helps, but i wouldnt want it near my prints.

its like running a loop with cracked acrylic tubes, they may not leak, but GOD do you not trust that.

Well I mean if your printer isn't under extruding, then you're melting layers of plastic together which should make a water tight seal. It might not be the most reliable thing ever, but with the right settings, I think it could be done.

the issue isnt that you cant print watertight, the issue is that you essentially have a point of failure every 0.1/0.2mm

Well as long as you don't put it under any pressure, it should be fine. The layers have a good bit of fusion in between them, that's why you can use printed parts in mechanical situations. You could also coat the inside of your part with some kind of sealing spray.

I did this back in 2013  Worked perfect with my test, but I used a insanely expensive printer with SLS technique

But you could use FDM with PETG filament, that would offer  a lot of creative flexibility. 

But you could use FDM with PETG filament, that would offer  a lot of creative flexibility. 

You will never get it waterproof with FDM.