I've tried to add a light to an underwater scene but the lighting does appear when rendered. Am I missing something obvious?

I don't think you're missing anything obvious, I struggled with this for awhile, too. It all depends on how you're achieving your underwater effects. If your scene is beneath a water plane, you can't necessarily rely on the sunlight to properly illuminate. Lots of spotlights might be one answer. Check out the underwater scene tutorials at rds' site: http://www.shoop3d.com/tut/tut.html I found those to take a unique and very effective approach to underwater lighting. I don't know offhand, but there may be more good help in the Vue Backroom. Good luck and let us know if you get good results! :-)

I too have struggled with this, one day I selected the water plane and added a glow to it....boy now that was something. Sharen

*Interesting.....*gotta try that. Thanks, Sharen!

Rick Shoop's tutorials are excellent. However, for quick underwater lighting try volumetric point lights at very low power, 5 -10. Also turn off lens flare. Gel effects can work quite well too especially dark greens and light blues. Note: placement is crucial and dependent on what you're trying to light and the effect you're after. A glowing water plane?! That's a "must try"!