The commercial diver courses I am familiar with also cover most of what an EMT-B class covers and beyond. You also have the advantage of being a commercial diver which consists of extensive gas laws and math knowledge.
Do you have the other requirements listed as training, including the NBDHMT CHT course, and experience?
You can also contact the NBDHMT and see if any part of your resume meets the requirements.
An EMT-B cert, which is only 110 - 130 hours in most states, which could be done in less than 3 weeks or even online with a brief clinical. The U.S. EMT-B may be not recognized in other parts of the world. The NBDHMT is just looking for a basic medical knowledge and first aid. In hospitals and wound care centers, RNs and RRTs will be utilized in the HBO centers for advanced medical requirement of HBO patients. Paramedic training in addition to the CHT education/training requirements would be great for offshore and remote locations but not always practical for a variety of reasons.