I think cold cathode tubes would work very well in this situation as well and they are readily availible at just about every pc shop. I use blue ones in my case mod. I'm not a red or green fan. It would be very easy to seal these for under water use as well as top mount. Any PC shop would have a used 200watt PSU lying around for free I'm sure. I do, if you need one. but this would give you the 12volt rail you need to run the cathode tube. This type of light will put off a flood style of lighting rather than a direct light of the LEDs. PM me if you want to explore this option further. I dig deep into PC's
I've read back through this thread and thought it might be a good idea to point out the reason we're talking about the color red for lamps or LEDs - octos cannot perceive red, so you watch a nocturnal octo without scaring it.