I am sure some here will have more in depth answers but Hazardous Duty Allowance is exactly as the name implies. To my knowledge, OP CARIBE is not eligible whereas OP METRIC was is (sorry REG). Levels of HDA are determined by the level of the hazard and the time an individual has spent in an HDA.
As I said, I am sure there is more answrs coming but I have to say, if you are in this purely for the $$$, I am a tad concerned. When screening sailors for AFG a few years back, one of the first questions I used to ask was why they wanted to go. If they said $$$, though the interview continued, my mind was made up...But then, that is just me.
Pat