I have the same camera with a Tokina 100mm lens. The first thing I notice is a problem I sometimes have. Your focusing on the base of the coral instead of the closest point. Make sure the closest point is in focus to fix that issue. Then you need to set the f stop high enough to get more of the coral in focus without losing so much light that the picture is ruined. I set mine somewhere between 8 and 12 depending on the coral. You will need a slower shutter speed and higher ISO to compensate for the f stop being higher. The picture preview on the camera will be very dark, but you can adjust it in Lightroom. Here is one of my photos. You can click on the photo and scroll down on the page to see my settings. There are a bunch of pictures on the site where you can see the lens and setting I used. Im no pro but this can at least get you started.
wd 2 031618 by
Big Jim, on Flickr