I don't know the camera you have, but it may have something to do with the minimum focusing distance. At any rate, the reason why you need to get close for underwater photography is because water is working against you. Being a differential filter of light, water filters out the longer wavelength light frequencies (reds, oranges) moreso than the shorter wavelengths (blues and purples). This is why things that are further away underwater tend to have that bluish hue to it (or green, depending on the water).
So, for underwater photography, it is ALL about getting close. Close means less water filtering the light. To get close, a good wide angle lens allows you to get close and still have a reasonable angle of view of your subject. This means less filtering. It also means less particles in the water, and as a result, the you get better quality of light hitting your film.