Hi Aquashot,
The main problems that I can see is that you really need to be in a position where the turtle is directly in line with you and the surface / sun-ball etc. as you are off to one side the sun will illuminate the top of the carapace and the silhouette effect will not really work. I think what you were doing / trying to do was correct and would have worked if, as you say, you were a little closer to the reef to get more illumination, you need some pretty powerful strobes to achieve a great silhouette and get colour from the reef unless you are really close in.
The title of the post is an often asked question, how far should you go? well it depends upon what and why you are shooting. If you are shooting for competitions then the rues regarding editing are usually pretty well laid down in advance. If shooting for an agency, for example, they have rules and regs regarding the use of filters even. Certainly if shooting for the US market, I have to declare the use of any filters that I use in my IPTC / Caption data when submitting images i.e. EDITORS NOTE; THIS DIGITAL PHOTOGRAPHIC IMAGE WAS CREATED WITH THE USE OF A POLARIZING FILTER or EDITORS NOTE, A COLOUR CORRECTION FILTER WAS USED IN THE CREATION OF THIS DIGITAL IMAGE.
Anyway, that aside, if you are using making the image for personal use then then you can do pretty much anything you want, however I think we all strive to get things right in camera and.
Just my two pence worth