I decided to have a crack at macro video on my GoPro because not every dive is filled with sharks and dolphins! I'm using a Backscatter Macromate Mini Lens which allows the GoPro focus at 3 - 4 inches (the image will be become blurry at shorter and greater distances). Accordingly, narrow FOV is the best setting for the GoPro.
So, put the lens on and stick it in narrow FOV and you're good to go? No, it's a bit more complicated than that.
You need light, but not too much. I'm using two Sola 2000 video lights on the lowest output setting which is 500 lumens. Too much light results in burnt images. The downside is that marine life and especially macro life doesn't enjoy being in the limelight and generally moves as fast as it can away from it. Also, I doubt that such bright light is doing these animals any good.
Stability is another issue. It is hard enough to get stable wide angle shots, with macro video, it is even harder. If I pursue this, I would seriously getting a small tripod.
Here's my first macro video after working this out. It's not great, but it's a start.
[video=youtube;xwcpQSep5zk]https://www.youtube.com/watch?v=xwcpQSep5zk[/video]