Fantasea sells several different under $200
video lights. They're on LeisurePro so you should be able to get them by next week.
Fantasea from LeisurePro
Optical Ocean Sales might be another option but their lights are more $$.
Focus/Video Lights :: Video Lights -
With the GoPro beam angle is important. It shoots 170
o so if you buy a typical dive light with a 5-25
o beam angle you're going to get a hotspot in the center of your video with dark edges. A diffuser really can't fix that so all you'll have is a less bright but visible hotspot.
Many video specific lights have 110
o beam angles so two of them provide optimal coverage. One is better than none also.
The brighter your light the more the Manta's will like you since you attract more of what they want (plankton).
The Aquasun might have a wide enough beam angle - IDK since they don't specify it
anywhere. The LED cooling heatsink in the center will likely cause a dark spot in your video though. You need even, wide angle light for optimum video.