First of all, the best shots are obtained by getting up close to your subject. After all, the closer you are, the better the fish is going to fill the frame. The color will be better too. However, having expounded upon the importance of getting up close to your subject, I need to point out that getting closer than two feet, even with the water correcting lens will result in a fuzzy, out of focus image. Good lighting is important to any phographer and this is especially true for the underwater photographer.
Tropical water acts as a cyan filter and quickly removes color from a scene, beginning with red.
In fact, without a strobe even pictures shot in three or four feet will show signs of color loss. There are several reasons why the flash that comes in the disposable camera is poorly suited to underwater use. Most importantly, because the flash is very close to the lens, back scatter can be a real problem. Moving the strobe up and away from the lens not only reduces back scatter, it places the light above and to the side of the subject. Because we are used to light sources such as the sun being overhead, a strobe in that position gives a scene a more pleasing, natural look.