I have heard the bioluminescence bay tours mentioned but don't know anything about them, just you tell us more about the tours and the experience, thanks!
Bioluminescence bay is located off Rum Point, on the North side of Cayman. We boarded out of the Cayman Yacht Club in West Bay, and crossed the bay - took about 20 minutes. Once in the bay, they anchored - it's about 5 feet of water, got a quick briefing, and jumped in. Just a mask and snorkel is all we needed.
As soon as we got in, the first thing I observed is the bioluminescence was about 2 orders of magnitude greater than anything I've ever seen on a night dive. The plankton also seemed smaller, and these were blue, not the green that I've seen on night dives. The best way to describe it, is when I pushed my hand through the water, it was like watching a Disney cartoon or something. Thick sparkly trails off my hand and arm. When moving my face quickly, they built up on my mask.
The best is most definitely when there is no ambient light - so a moonless night, or in our case using the shadow of the boat and the tarp they provided. Once in this darkness, the bioluminescence becomes that more visible. Without any motion, just gentle drifting through the water, these little pinpoints of blue light just swirl around your body. We could have stared at it for hours.
The boat tours are better, as you don't actually swim on the kayak tours. Don't bother with photos. Our photo pro brought his Canon 7D mark II and said there was way too little light, even with half second exposures and the ISO on HI2 (51,200.)
We went with Georges Watersports who was very good.