The Kona manta dive is strictly a night dive with in-your-face manta encounters. You will have mantas passing often within inches of you multiple times throughout the night, necessitating wide angle. Night-time wide angle can be tough, especially with only one strobe. I would recommend trying a few ideas to see what works.
-First, you could try a high ISO with no strobe, utilizing the abundant ambient light. My inclination is that you might have some success as the mantas pass over the central "campfire" of lights.
-Use your strobe way out and back to the side to minimize backscatter. Then wait for a manta to come in close. I would shoot in manual exposure, iso 400, 1/100 sec, low f-stop.
-Your best option is try to coordinate with another diver with a slave strobe that is triggered by the flash. This is basically like having an off-camera second strobe. Sit next to them and experiment with a few angles to figure out what works best. You could get really creative with this one, but I'd start basic and work your way there.
-Finally, utilize a few dive lights and shoot some video. Shooting stills of mantas at night can be really tough, especially if you don't have a second strobe, so often those without strobe#2 get better results with video. The first time I did that dive I only owned one strobe and was severely disappointed in my still-results but the video I took came out awesome. Years later I came back with a second strobe and nailed it. (results found here:
http://milisenphotography.yolasite.com/manta-rays.php).
Whatever you decide to try, good luck and please post your results!
