Welcome to WWN, patternspace. I tried 3D printing back in 2009 or so for a Nerf project, and was not happy with it.
I'll second HBWW. 3D printing would be a poor choice for the strength. The surface finish is also pretty bad. This means two things. 1. Sealing things to 3D printed parts requires post-processing (sanding, or epoxying on a flat surface, for example). 2. Turbulence could be generated from the surface roughness.
The strength problem might be overcome from using thicker pieces. But CNC would be better. The same is true for the surface finish.
RyanMcNumbers at NerfHaven has made some nice handles and structural pieces with 3D printing, so if we ignore parts under pressure or that have flow through them, we definitely can make some progress. There's still a problem: Making large pieces like these would be fairly expensive. (They might be worth it, still.)
In my experience, most people have unrealistic expectations for what 3D printing can provide. While it might be becoming more popular, its quality leaves much to be desired and the price has yet to drop into a reasonable range. So I don't think this is so valuable at the moment.
There do not seem to be so many people who use 3D modeling software in the water war community, but in the NIC you should find plenty. And I think the parts needed would fit better with what 3D printing can provide.