How Low Can You Go?
Nigel uses a Canon 50D, a Canon 300L f/4 with a TC 1.4. He doesn’t need any stabilization… In this case no tripod… Just a small piece of foam underneath the camera and on the side of the lens to make sure that the lens doesn’t end-up in the water.
Here are some of his photos on Flickr: https://www.flickr.com/photos/winnu
I have only one word: Amazing!
BTW, Not too long ago, Nigel upgraded to a Canon 5D MkIII with a Canon 500L/f4 and the TC 1.4mkIII.
Technical
Camera: Canon 7D ISO 400 and 1/250
Lens: Sigma 120-400 f/4.5-5.6 @120mm and f/8
Processed with Darktable 2.0.5: https://www.darktable.org/