Hey Eric,
As long as I have been publishing websites I've been fighting copyright infringement. Comments here are right on the money with the posted suggestions (and thanks for the Rick Rickman links too). I've collected very similar copyright settlements without ever involving an attorney.
I've also blown-off several million dollars worth of potential income by determining that the person who stole my images (or in some cases, entire websites) wasn't worth going after. In all cases, I always contact the infringer with the nicest, most professional legally-worded letter I could possibly muster up in the heat of my rage. I've learned that nice letters don't result in payment. At best, you'll get an apology. Worst case, you're called every name in the book and might end up at the bottom of the ScubaLinx photographer ratings. In one case a well-known dive travel wholesaler from the Chicago area ultimately threatened my life!
The only people who give a squat about infringing your copyright is the legal department from the big company who did it, because they know the law, and the consequences. If you let them know that you REALLY understand your legal options, and this is WHY you register ALL your images with the copyright office BEFORE you release any of them to the public, they will know you mean business. In cases where I collected settlements, I was persistent and DID NOT back down in spite of the complaints that my 3x normal usage demand was excessive. By the way, I could find no legal backing for this demand. It seems to be an agreed-upon industry standard settlement in leu of legal action. Some ASMP photographers demand 10x, but I don't know if they actually get it.
If you had already registered this image with the copyright office before you ever published it, you're in luck. You've got a lot more legal power than if you didn't. If you did not register the image with the copyright office, do it now. Not only is it important, but it sends a signal to the infringer that you're planning to take legal action (whether you are, or not). You cannot initiate the process until the image is registered with the copyright office.
A couple of other suggestions... get Cradoc FotoQuote's (http://www.fotoquote.com) newest version of the software to build your usage estimate. The fees are higher than older versions. Be sure to add in for the fact that underwater photography is in a higher-risk bracket than say...wedding photography. Also, consider long-term consequences of any of your actions. Even though you're right, you'll be burning bridges forever if you go public with their name. Until you're absolutely sure who's responsible for the release of your image, and know for sure that they have no intention of making it right, I personally might hold off on the exposure part. I know this is hard to do, but I've also picked up some long-term clients over being "nice" about "misunderstandings" in certain situations.
Now the disclaimer... I'm not an attorney, nor do I play one on TV. So if there are any copyright attorneys out there who find what I've written here is inaccurate, please jump in and correct me. I'm just blabbing from my experience in hopes that it will help other poor, starving artists to earn a little income to pay for this horribly expensive hobby some of us call a job.
Hope this helps!
Bonnie