Getting that iconic manta belly shot can be a lot harder than it looks, at least for me. It's more than just swimming under a manta and snapping away. There are many different factors to consider.

First you have to have to be able to get under a manta which can take a bit of patience, an understanding of manta behavior, and precise timing. Your best bet at getting under a manta is either when one is passing over head on its way to somewhere else or in which case you need to be quick because you will only have a few seconds to frame your shot as it passes over you. The other way is to find a manta that is staying in one place and just wait a few meters away so that it can see. Mantas are curious by nature so as long as you remain calm it will most likely come give you a good inspection.

You’ll want to have a quick shutter speed and small aperture combination to give you that nice dark blue background. You will also need to consider your strobe power which should be on full so as to illuminate the belly of the manta properly. It also helps to have a smaller manta so you can be closer, improving image clarity and allowing your strobes to really reach their full potential. If you have a large 5 meter manta it's really difficult to illuminate the whole thing unless you have a set of cannons for strobes, as you need to be further away to fill the frame which means the lights from your strobes is also at it's weakest point.

As you will most likely have your lens angled upwards, watch out for your bubbles. As we all know, bubbles are an underwater photographers worst enemy, they detract from any image significantly and will often times scare mantas away. Controlling your bubbles comes down to timing. You will need to time your exhale so that when you snap the photo the manta is directly over the top of you and your last bubbles have already disappeared out of the frame.