as you are moving (swimming) when you shoot - improving apparent focus or sharpness usually helps
Photo edited without sharpening
'Sharpening' a snorkelling
photograph improves apparent focus and gives a big gain. Can boost even after
'clarification'. Photo above has clarification of 25%. Note the eye before and after.
You will have been moving. The fish may have. The light is less good than on land and
the camera can be confused as to exactly where you are trying to focus if the subject is small or moving.
Photo edited with sharpening
Sharpening cannot fix a blurred, out of focus shot. It improves rather than fixes. If your
software offers 'unsharpen mask' it works well. Otherwise sharpen by whatever is on offer.
Most advise to resize your photograph before you sharpen, especially if you are going to print
it. Essentially sharpening is the last thing to do in the photo editing flow.
The first left-hand picture below is:- + colour balance + 10% contrast + 10%
vibrancy + 10% saturation.