Is your light above the gobo, so that the light has to pass through the gobo to reach the water box? the gobo should be above the water box, but below the light; generally, you'd want to use an infinite light, rather than a spot, area, or point light. Don't have other lights in the scene, as they will wash out the godray streaks. I do have other lights in the shot I posted, but they are used in such a way as to avoid destroying the godray effect.

Poser 12, Poser 13, Poser 14, in feet.  

OSes:  Win7Prox64, Win7Ultx64, Win11

Silo Pro 2.5.6 64bit, Silo 2025.1, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5