My painting technique is a series of transparent washes built up on layers of opaqeous acrylic. I feel it is important to have a far background, a midground and then a front foreground to add interest and depth.
Because the subject matter you are painting is underwater, it is important to add washes of blue over the whole painting, especially in the background to make it look farther away. Remember, you are looking through water to see your subject matter and surroundings. If its too detailed, it won't look appear as if its underwater.
I am currently working on a dvd painting series for the underwater world.