Are you thinking of the Ostracod displays? If so, you want to be looking about 45 minutes after sunset, not near a lot of shore lights, and when the moon is not up. It needs to be dark, and your eyes need to be dark-adapted (no lights are used). Since the Full moon rises at the same time the sun sets, and then a bit less than an hour later each night following, the two weeks following Full moon (but not the 2-3 days after Full moon) work well, but the earlier part of the two-week window is the best. Apr 29 is Full moon, with sunset at 2012 and moonrise at 2000, so it is not dark that day, nor is it dark enough the next day, But by May 1 you've got sunset at 2014, real darkness at 2100, and the moon not up until 2156. Then you've got some days of good activity.
If you stick with your 5/5-5/20 dates, you'll be in good shape immediately upon your arrival, but it is likely to peter out in a week or less. I think they get tired flashing! I'd go with the early arrival!
If you are thinking classical bioluminescence, as from plankton in the water, it just needs to be dark.
Your profile says you are a photographer. As far as i know, you need an image intensifier to photograph the Ostracods, and probably the plankton displays too.
Your profile says you are not certified. Seeing the Ostracods as a snorkeler is problematic. Many have tried, but the problem is the activity is down around 25-50 feet, and the light is quite dim, so from the surface there is not much to see.
