The D5100 hasn't been out for long. It takes some time for the housing manufacturers to get their hands on a camera and then construct a housing for it. Given the fact that the combined costs of the ports, housings, lenses and strobes are substantial, I don't know if it makes sense to go with a D5100. The D5100 lacks its own focus motor which is important. Another thing it lacks (or so I understand) is a dedicated ISO button. It can be really handy to adjust your exposure in manual mode by adjusting the ISO some. Having ready access to the ISO control means that you can control, ISO, Fstop, shutter speed and flash intensity to adjust your exposure. I would think that a D90 or a D7000 both of which have the ISO button would be better candidates for underwater photography.
For housings, Ikelite makes the most inexpensive quality housing. It does have some drawbacks. Ikelite's housings tend to be large and their controls less dialed in. The way Ikelite makes a housing is take a standard box and fit in the connecting controls to the camera from there. The aluminum housing manufacturers have to custom make each and every housing so their housings cost twice as much as Ikelite. When you are buying lenses, strobes, ports, focus gears, focus lights etc, the more expensive housing might not add that much to the price of the system. So it is something you might want to consider.
