I just started with my 4600 and the Nikon housing and quickly discovered that a strobe is a must. In deep water, everything has a blue cast and often in shallow water, the internal flash didn't auto-flash and the pictures were washed out. I used Underwater mode for most of my shots. I plan to try it using manual adjustments next time. My Coolpix just doesn't allow for many manual adjustments.
I think it is an adequate camera for learning underwater photography. I'm spoiled in that I've been using a 7 megapixel Sony DSLR for a couple of years. I bought the Coolpix with the knowledge that if the camera flooded, it would not be the end of the world.
If you look in my gallery, the photos of the wreck (with the blue tinge) are taken with the Coolpix and the more colorful ones were taken with my buddy's Cannon 500SD using only the internal flash. Really big difference in the results.
I'm going to buy the Sealife 960D strobe, which supposedly works on all digital cameras so that I can reuse when I upgrade.