Anthony White ran some experiments with 3D printed ABS parts. Here's what he had to say on Google+:
What happens when a printed part is exposed to 3000 meters of water pressure for 48 hours?
not a whole lot. The part is porous enough that water forces its way in, eliminating any air pockets, and filling the structure. The part on the left, which was exposed, is even smoother then it's companion (they were smoothed in acetone vapor together.) The larger pulley part I tested was a bit waterlogged but doesn't seem to have any significant reduction in strength!
Thanks to Reimer for originally posting this to the OpenROV G+ Community Page.