You have to get one. It's a longish kind of process but it's not too difficult once you get organized. Here are the basic steps. A more detailed guide will be available in the coming weeks but for now, use the message boards for advice if you get stuck.
First. Go to the USCG site and find the link for licenses and documents.
Here's the page to get started.
After you get your STCW certificate, begin this questionnaire again. This time around, you should get the job you need.