Hi all, to give you a little background, I'm a plastic surgeon who has had experience with treating all kinds of infections, especially infections of the hand.
Capn, the mechanism of your injury set you up for the infection. A puncture from an urchin's spine is a wound that has a deep tract in comparison to the surface area of the wound at the skin's surface. Even if you wash the surface of the skin, you are not going to be able to remove the bacteria that has penetrated deeper due to the nature of this tract. Second, the puncture probably entered the subcutaneous fat on your hand which is relatively poorly vascularized (has fewer blood vessels in it) in comparison to the overlying skin itself or the deeper muscles. This makes it harder for your body to deliver white cells to the injury to deal with the infection. This allows the bacteria to begin to reproduce relatively unchecked and sets you up for the infection. Third, if this particular species of urchin is venomous, the venom will likely cause the tissue along the wound tract to become necrotic (dead). Dead tissue is a safe haven for bacteria and they will grow quickly in it. Fourth, the local environment of a puncture wound is not exposed to oxygen in the atmosphere and is what we call "anaerobic" so certain species of bacteria are able to grow unimpeded. Fifth, if you have recently undergone or are undergoing treatment for cancer, it possible that the medications that you are taking have caused some immunosuppression. If this is the case, it is unlikely that you would have been able to deal effectively with this type of infection. You would need to talk to your doctor about this. If you are immunosuppressed, you could have further trouble with other types of infection. This doesn't have to do with the lymph nodes that were removed during your prostate operation. Operations where lymph nodes were removed from the arm pit may increase your risk of infection in the hand though.
In terms of the offending organism, I wouldn't bet on Staph or Strep as the source of your infection. There are a few others that are more likely given that your injury occurred in what is basically sea water. These bacteria cause significant limb threatening and even life threatening infections in people every year. In your case, I would probably bet on Erysipelothrix rhusiopathiae. If you want some quick information on this or on other marine infections check out:
http://scuba-doc.com/marinf.htm
This summer here on the Texas coast, a few people lost limbs or died due to Vibrio vulnificus. These infections occurred in people who were immunosuppressed.
-B