How can I make a particle system that does not face the camera and is constrained to face the Y direction. I want to create bubbles on the surface of a water plane. Please forgive me if I missed something obvious. 
Unfortunately, the particle renderer built into Unity only does camera-facing billboards. For anything else, you’ll have to script it yourself. Luckily, simple particle dynamics are easy to model. Just keep a velocity for each object, and apply forces every frame.
Actually, I just thought about this some more, and if you’re going to be rendering a lot of particles (thousands), you might want to look into making a mesh and modifying sets of four vertices using the mesh interface. This will be a lot faster than moving separate objects around.
Darn, I was hoping I just missed something. I may have to try one of your methods. Thanks for the info.