Most cases there always is NOT enough light, so by default flash is ON.
In cases where you want to try black and white where there is no color anyway, just gray scale, you might as well turn off the strobe. Shoot the photo just like that.
If your are trying to shoot wide angle stuff with your Mju, there is no way the internal strobe can reach or cover the distance or spread, again might as well turn off the strobe (there always will be an exception).
Photo 1, I guess you took the photo shallow with great lighting. No scatter and dull colors so probably strobe was OFF.
To improve photo 1:
The fish are a bit blur, this means the speed is too slow. If you want nice sharp fish that are in constant motion, you need to somehow use high shutter speed. This means 100 and above, in this case probably 125. If your camera only has auto something, then choose sports mode.
Next advise since the bottom part has coral and the tiny strobe is at the top of the camera, turn on the strobe and shoot the photo holding the camera upside down. The bottom coral will catch most of the light and block it from going up. This also helps minimize back scatter. The coral and colors also come to life.
Photo 2, strobe is on and too far away.
To improve photo 2:
Get closer and take an angled shot of the feather star with the coral in the background. You can try the upside down technique again, but do not think it will help in this circle object case.
Photo 3, strobe is on and too far away.
To improve photo 3:
Not easy to get closer to the stingray. I would try and follow it and wait for it to rest. The slowly approach from the front and get a nice tight shot of the face instead of side.
The tiny strobe of the camera simply has very little power, so it cannot reach the main subject. It only has enough power for maybe 1 meter or less distance underwater. So you have to choose a technique and style that suits your camera.
Your first shot was actually not too bad, but try it upside down with the flash and macro on.