In my experience... typical 3D printing is good for structural, planar parts but as it's built up in layers is poor at staying non porous or dealing with twisting loads. Although I can print solid I reckon you'd want to do further finishing before you trusted even a small part to stay dry inside - that might include vapour smoothing with something evil :-) That's just to get the body of the print less porous, surface finish would need work to mate faces and o-rings too.

It would be great for prototyping but you'd have to overengineer and process prints... whereas machining solid material should get you waterproof for free :-)

Rob