I was shooting the Nikon D90 before I moved onto the D7000. I find the D7000 alot better at capturing sunbursts mainly due to its flash sync of 1/320th with my hard wired Ikelite DS161 strobes shot in manual.
The Nikon D90 can obtain decent sunbursts but there a many viables which need to be taken in mind as its not just a matter of camera settings. I agree with Morten that low ISO (Lo 1 on the Nikon d90) and high shutter speed. It also has alot to do with what your trying to achieve in the shot. Are you just after the sunburst or are you after the sunburst as a secondary importance to a subject like a model or animal (ray,shark, turtle etc etc) as this also influences your settings.
As Morten stated sun position, surface conditions play and important role but also one of the most important factors is depth and water clarity. Most of the successful sun burst I have have taken have been in shallow clear water with still surface conditions. I beleive once you gain greater depth that you will end up with more of a sun ball glow than a sun burst as the water has some form of colour (unless gin clear) which acts as a filter/diffuser to softens the light. The deeper you go the more is effect comes into play. It also the same if shooting on a cloudy day as the clouds themselves soften the light and act as a diffuser similar to whats on your strobes.
My suggestion for you to achieve high impact sun rays is to shoot as shallow as your subject allows, pick the day were the surface conditions are calm and the water clarity is high. Lowest ISO on your camera (lo 1 on the Nikon D90), fast shutter speed and a f stop relative to what effect your trying to achieve.
If your diving deeper water you can still use the effect of the sun for a warm ball feeling as a background to your subject by positioning the subject within that sun ball.
Here are some examples:

Nikon D90 Tokina 10-17mm @ 10mm. ISO LO1, f10, 1/200th

Nikon D90 Tokina 10-17mm @ 10mm. ISO LO1, f10, 1/200th

Using the subject to cover part of the sunburst.
Nikon D90 Tokina 10-17mm @ 10mm. ISO LO1, f10, 1/200th

Using the Sun ball at depth. Nikon D90 Tokina 10-17mm (@10mm) Ikelite DS125's. ISOLO1 (100) f13, 1/200th
Hope this helps.
Regards Mark