Camera system with versatility. Wide-angle lenses will provide the best all around images (focal lengths of 35mm - 20mm), however beautiful portraits will be best achieved with a slightly longer lens such as a 60mm.
Two strobes will create better lighting, especially when working close to the subject. Ideally you will meter for the background water (ambient light) and then use strobes to fill in the shadows. Sea turtles are not especially big animals, but some can get pretty large. Make sure your strobe's angle of coverage will light your subject well. Two wide-angle strobes (with 90-degree angles of coverage or greater) should guarantee even lighting.
Strobe arms that allow creative positioning.
Focusing light or strobes that feature modeling lights.