Hi, I followed, I thought, A tut over at Geekatplay for a simple underwater scene. I used a quad spot with volumetrics turned on over the water plane, and it did light up the sea bottom. I then added a caustics light gel to the spot, but nothing happens. The caustics pattern does not show up. What am I doing wrong? Thank you, Sherrie

I have the same problem ......

you might try adjusting the scale of the caustics gel on the light (reduce probably)

Reduce the gel's size as suggested by FrankT, and also, use a brightness/contrast filter node in the function editor to get a pure black and white gel. Grey values don't show much in the light. I know this is not easy for a beginner, but do as follow:
Choose your gel, Vue will open the light gel tab of the light editor, right click the gel preview, and click the link between the gel and the altitude input. The link should now be thicker, then click the filter icon on the left toolbar to add a filter. In the drop down list of available filters, choose, brightness/contrast, then add contrast.
In some cases, there are laready some filters between the gel function and the altitude output, just add a new one, and don't touch the ones already there.
I don't find the "ready made" gels very good, and rely on this software to create the caustics patterns:
http://www.dualheights.se/caustics/

Did you delete the sun? It could happen that the sun is so strongly iluminating the scene that the light from the spotlight is not visible enough.

Thanks so much. I will try this. Didn't know I had to delete the sun.

I don't know if that's the solution, it was just a hunch. I didn't see that tutorial and so I'm not sure what's your lighting configuration. If, for example, you already had the light intensity slider with a low value, then deleting the sun won't help much. In any case, it could help to have a better contrast (and therefore see the gel effect) if you don't have other lights.
Good luck! :-)

This may seem obvious but I'll mention it anyway.

I played with another scene and found that my light power was set to 100
I turned the power down to 10 then I can see the caustics pattern

Attached Link: Liopleurodon Image

I do quite a bit of underwater in Vue. Currently using 7.5
I've never deleted the sun, for what it's worth. A couple of tips: Make sure your volumetric light is above the water surface. I also find that angling the light makes a big difference in the caustic effect. Agree with the above comments about making the scale of the gel smaller and also increasing the contrast, though rather than going into the function editor, I've done it by going into the color map and adjusting the slider. Also, increase the volumetric light's spread can be effective.
The other thing is that you may need to have more than one light. In the attached image, I used two volumetrics to get the light beams, then another 3 lights non-volumetric but with gels attached at a very high level (300) to get the light patterns on the animals. These non-volumetric lights can be placed below the water surface.

That's beautiful. I will try that.