3D printing certainly has promise for our hobby. Unfortunately, it is probably not as useful as most of you all imagine. I think there are good possibilities for some replacement parts in stock water guns, but if you want anything that seals or has good surface roughness properties (rougher = more turbulence), then you should look at alternative manufacturing techniques. I used some 3D printed parts for a homemade Nerf gun
back in 2009. I found that I could not get a seal with the parts and had to epoxy on a flat polycarbonate sheet where I wanted a seal. I've read you can sand parts to get the right finish, but this would be a fair amount of work for a water gun made largely with 3D printing. I think alternative machining methods are worth considering. CNC milling holds a lot of promise for custom nozzles (the surface would be very smooth). A few spud gunners use epoxy casting to make valves and I think those methods could potentially lead to valves with better ergonomics and good surface roughness. Likely you'll have to match the part with the right machining method.
Another point is that 3D printing is usually very expensive (same for CNC milling, etc.). The small parts I made back in 2009 cost me about $45. I imagine a 3D printed valve would cost more. A 3D printed shell would probably cost hundreds of dollars! If you make parts in bulk, the price could go down, but I'm not sure we'll have enough interest to make much of a dent. Price-wise, 3D printing is probably okay only for small parts like replacement triggers.
In contrast, epoxy casting is relatively cheap. If you could make a mold for a trigger then it would be cheaper than 3D printing. I don't know a whole lot about this, but Google could take you far if you are interested.
One last point I want to make. There are online services that do 3D printing and CNC milling
. The advantage to a local shop is that the machinists, technicians, and engineers in shops are really awesome people who can offer great advice. However, in terms of just being able to make your design, you can get the same results for about the same cost (maybe cheaper, but still expensive) with an online service.
I could be wrong about these points, but this has been my experience with 3D printing and I think it's worth sharing.