Figuring out how to get underwater photos published in diving magazines is something that you will end up having to figure out if you ever want to become professional or semi-professional at it. Doing this is very similar to getting recognized for any artwork. Exposure is a key feature of this process. The best way for you to do this is to make your pictures available on social media that focuses on user created content like pictures. Facebook, Flickr, Reddit, and other sites that allow free posting can be used to gain attention for your work.
Since you do not want anyone to use your work without permission, it is important to put a watermark on your picture that labels it a sample. You can even use your email address or website name as the watermark text so that anyone interested in getting in touch with you can do so easily. You can also take a more direct approach and compile a portfolio that you can send to photo editors at magazines that you are familiar with to view your photos.