The problem with FDM (Fused Deposition Modeling) 3D printing (most open source printers, and Stratasys) is that with the layer upon layer printing, you get a thinly-stratified surface finish which makes the parts structually weaker, and enables liquids to ingress into the part.
I came across this method on the rep-rap blog. It gives FDM 3D printed ABS parts a nice annealed surface finish which is almost injection-molding like. I plan on getting a 3D printer soon so I can test making parts with this method, and pressure testing them to see if they can be truly water-proof. In my experience with FDM printers (Ultimaker, Replicator), printing with 100% infill with a bit of tweaking with the extrusion rate can make fully infilled solid plastic parts with no bubbles (as far as I could see).
Has anyone tried this method with 3D printed parts for underwater use? I imagine this could be really useful for making waterproof liquid compensated enclosures and the like.