I recommend that you don’t book Captain Marvin’s Bio Bay tour because they operate on nights where the bioluminescence isn’t visible.

In this article, I’ll explain how to know when the bioluminescence is visible, and which tour companies operate on these nights.

 

What is the “Bio Bay” in Grand Cayman?

The Bioluminescent Bay is a bay near Rum Point that was dredged out over 30-years ago as part of a real estate development.

The physical properties of the bay allow microscopic plankton, known as “dinoflagellates“, to multiply. The narrow entrance of the bay prevents them from escaping, so their concentration increases over time.

These dinoflagellates emit light at night when disturbed in the water. Due to the high concentration, there is a blue/green effect.

 

 

Why you shouldn’t visit the Bio Bay with Captain Marvin’s

Captain Marvin’s operates their bio bay tours on dates where the bioluminescence isn’t visible.

They do this to increase their revenue, despite knowing that their customers are not going to see the bioluminescence.

The only time that you can see the bioluminescence is when the two things are true:

1. You are in the Bio Bay no sooner than one hour after sunset
2. The moon is not in the night sky at the time of the tour

You can only see the bioluminescence when it’s very dark, and if both of the above factors are not true, then it won’t be dark enough to see it.

A common misconception is that people think that it needs to be a new moon (not a full moon) to see the bioluminescence, but this is not necessarily true.

What actually matters is if the moon is visible in the night sky at the time of your tour.

For example, it could be a full moon (100% illumination), but if the moon doesn’t rise in the night sky until after your tour finishes, then it will be pitch black, allowing you to see the bioluminescence.

If the moon is in the night sky at the time of your tour, then you can only see the bioluminescence if the moon’s illumination is less than 30%.

The problem with Captain Marvin’s is that they are operating bio bay tours when the moon is in the night sky during the tour, with an illumination of more than 30%.

Below is a screenshot that shows their bio bay tour availability on their website; it shows them operating bio bay tours on the 30th December 2025, when the moon illumination is 84.3%. The moon rise time on the 30th December 2025 is 2:12pm, meaning that the moon will be in the night sky at the time of this tour:

They are operating bioluminescence tours with full knowledge that the bioluminescence won’t be visible.

Even at an illumination of 30%, the bioluminescence is greatly diminished, so at higher percentages, it’s completely invisible.

 

Which tour companies only operate on visible nights?

Most tour companies only operate bioluminescence tours on nights that have good bioluminescence visibility.

George’s Watersports only operates when the bioluminescence is visible.

You can also check this website to verify that the moon rise / set times are correct.

 

 

Final thoughts

I recommend that you don’t book your bioluminescence tour with Captain Marvin’s as they operate tours on nights where you cannot see the bioluminescence.

Instead, you should book with a different operator (like George’s Watersports) that only operates when the bioluminescence is visible.

On a related note, if you want to learn more about the bio bay, then check out my article Bioluminescent Bay Grand Cayman – All You Need To Know.