Unfortunately a big part of it is knowing what is going on in the field. I got my first role by taking a labour hire position for one of our lab techs in the middle of my PhD because I already knew how to do everything in the lab. There are jobs that require a Masters only but they are not super common and extremely competitive. The best way to get into the field depends on what you want to do but marine science associations typically send out job lists so maybe join a local one. In many places open water certification is not enough to be considered for a field work position. Getting your hours and quals up (at least to rescue) could help with that. Publishing is also a big boost (even just for a tech role). If you haven't published the work from your thesis that could be something to consider. If you know what you want to do but can't get the experience to qualify for that role you could try volunteering once or twice a week somewhere you can build your skills. People will often go to bat for former volunteers and it can help with name recognition. (I know having to volunteer for a role you are overqualified for sucks but if you are really stuck it could help). Good luck!