Looking for a bit of advice for how much of an ISO change would be recommended for a full stop change of F/4 to F/5. I will be shooting in natural light. The attached picture is an example of what I would like to work on for the next shot, just want to go into the next dive somewhat prepared as the opportunities are not that often. I am mostly happy with the result after some work with Lightroom, however you can see there is a bit of softness on the lower left fin of the turtle that I am hoping a move to F/5 could do away with. Or if I should be trying something else all together. For simplicity let's just assume that the lighting conditions are the same or similar for the next dive. Here's the info from the example photo. Also a side note, I cannot adjust the shutter to anything less than 1/200 for the types of shots I'm looking for. Thanks!
Shot on Canon 5D Mark IV - 24mm 2.8 EF IS lens - spherical dome port - natural light
F/4 - ISO 500 - Shutter 1/200 - Manual white balance
45 feet deep, mix of sun and clouds, sun at back