The image does not look too blown and would have been workable from raw. You will need to do a lot of post processing shooting a turtle from a distance (> 1 m). At these distances the strobe will be contributing relatively little light and at 10m depth the light is already rather blue.
Have you tried working with the jpg? Some basic jpg image editors (such as Preview on a Mac - it is under "adjust colors") have white balance control and contrast control.