Faster shutter speed will prevent blur by stopping any camera or subject movement. It will also darken your whole picture. That shot suffers from too much light in the distance, because you're aiming right into the sun, and the surface is brightly lit. The foreground is well lit, because it is closer to the camera in deeper water with less light. By upping your shutter speed, you will make the whole picture darker. This will improve the overlit distance, but cause a dark foreground. You can add light to the foreground with your strobe to make up for this. Higher f-stop (f8,f11,f22 etc., varying with what your camera is capable of) will do the same thing by decreasing the size of the shutter opening. Generally, this will cause a dark background with a well lit foreground where your strobe is aimed. A side effect is increased depth of field. A wide open lens will blur at all but the correct focal length. A small shutter opening gives a wider range of focal length. This can help on macro shots, where you're really close to the subject. If I'm being a complete idiot and have this all backwards, I'm sure one of our photo pros who is lurking will correct me.
BTW, I should have noticed the clamshell! Being closer always helps because the color is better. Everything turns blue in the distance.
