Remember that speed is everything. Too much speed is a waste, but having too much to eat is better than starving.
The example I gave you to start with was a deliberate example of too much speed. Now you can bring your ISO down for less interference, or bring your aperture down for more DOF (how much of the coral is in focus). The sharpness of the image will be best in the f/5.6-f/8 range. Wider than f/5.6 or narrower than f/8 will (slightly) reduce the quality of the image.
You have to keep giving and taking until you find the perfect mix. Your first step should be to find EXACTLY what shutter speed you can get away with in your situation. 1/20? 1/30? 1/60? 1/100? Once you know your minimum acceptable shutter speed, you can spread the excess between ISO and Aperture. In macro work, less aperture (f/32 vs. f/2.8) is often better. This is simply because more of the subject will be in clear focus. If you only want a specific portion of the subject in focus, a wider aperture (such as f/8) may be the best option. So you pick your minimum acceptable shutter speed, pick the aperture you need, and the rest goes to ISO.
Take special care to observe and more importantly understand the exposure meter in the bottom of your viewfinder when working in manual mode. The needle which fluctuates between -2..-1..0..1..2 determines how dark or bright the camera thinks the image will be. The -2, -1, 0, 1, and 2 all represent "stops". Remember that vocabulary word? The periods in between the numbers represent 1/3 of a stop. There are several metering modes which will change this reading. For example you can meter off the entire image equally, a small portion in the center of the image (the circle in your viewfinder), or other less straight-forward modes. The metering mode you select will possibly change where the needle rests in the exposure meter. This is one reason manual mode is superior over other modes, and why full automatic is not the way to go.
In manual mode the increments of the exposure will never change unless you tell the camera to change them. If you take pictures in automatic mode, the images may vary from too dark to too bright while still staring at the same subject! This is very evident in panoramic photographs done in automatic modes. I know a guy who posts panoramics on Myspace all the time from his dedicated "Photography" page. He refuses to actually learn photography and you can easily tell where the exposure changes within the shot to the point of being ridiculous.
What I am getting at is that the camera does NOT know best. Just because that needle is on the center at "0" doesn't mean it should be. If what you are metering off of is white, the meter will underexpose and try to make it gray. If what you are metering off of is black, the meter will overexpose and try to make it gray. With experience, and only experience, you will learn instinctively when to purposely overexpose or underexpose. For example, green trees are relatively dark in the world of photography. The camera will normally, and incorrectly, overexpose +2/3 of a stop to correct so you have to counter this and underexpose -2/3 of a stop to compensate. The result is exposure bliss.
I personally prefer "pieces of subjects" shots v. "whole subject" shots. You might try cropping tighter on an interesting area in that shot.
