I think the most important part of shooting a sunburst (sunball with sunrays), is recognizing the conditions that it will happen. The rays themselves are not at all the brightest portion of the shot, by orders of magnitude - as a result, as one goes deeper, the chances of capturing the sunrays diminish. Additionally, the rays are often created by a mere turbulance on top of the water, a simple swell action. It is important to keep an eye on the surface on occassion - when the rays appear, you will know.

1/400, f/18, 18-55mm @ 21mm
This jellyfish was shot extremely shallow, perhaps at 10-15ft depth. Notice that the image exhibits the hint of possible rays, but degrades into a sunball both because it was allowed to bloom a little more than normal, as well as the surface conditions.

1/320, f22, 18-55mm @ 33mm
I'm not sure what category this falls into. The sunrays are not at all like they usually are - one individual described the wisps and tendrils of light as "god's fire." At the time (while shooting kelp), I noticed a change in the light coming down, and looking up, saw dancing rays on the surface. I feel this is the best time to shoot for a sunburst. It does help if there is some manner of particulate matter in the water for the light to reflect off of...in the clearest blue tropical waters, you may never record sunrays, or only very rarely.

1/250, f/18, 18-55mm @ 18mm
Silhouettes, or partial silhouettes, are perhaps the easiest to do against the sun. Notice however that it is not always necessary to use the subject to block the sun. In some cases, this may be impossible - in an extremely bright day, the subject can be lost inside the glare and washout caused by shooting straight at the sun, even underwater.
Make sure to remember that you may be limited in your exposure choice if you want to add strobe -lit foregrounds. The previous photos were all ambient light.
Digital sensors have reached a point where they too can capture the finer details of a transistion from the deepest blue of the ocean to the white overexposure of a sunburst. It's up to you to experiment...