Turn your hobby into a profession without losing control of your photography copyright by becoming a publisher. Learn to become a web publisher and start to publish your UW photos on your own web site.

The old fashioned way would be to try selling your UW photos - to surrender your rights to a photo agency and wait until your few % drops in.
The modern way is to start publishing on your own web site. Convert your truly beautiful Underwater Photos into wallpapers in standard format, in widescreen format or in mobile format as well as create e-Cards from all your UW photos.
Modern and far more spiritual methods are to share all with all - the revenue comes from advertisement revenue such as Google ad sense or other advertising agencies. As you may have noticed, as of the time of this publishing I use Google adsense as my advertising provider. For a reason! The large network of ads acquiring agents across the planet offers a huge inventory of ads for all different topics / keywords and as a result you almost surely get precisely matching relevant ads that actually add value to each page by offering value added additional web sites or services relevant to each and every page of your web site.
As a result of this true value added advertising scheme by Google (and others as well meanwhile ) you may expect a percentage of 5-15% of your site visitors to use one or several of the ads displayed. The result is that as a web publisher you start to earn as soon as your own self created advertisement platform is being used.
Prices for ads are by bidding - hence there is no fixed price and the market value or demand may vary during the course of the year, by country and of course by precise topic you publish. My experience as a web publisher is that SCUBA diving is far above average for eCPM / CTR. i.e. for $ per thousand page impressions as well as % of site/page visitors clicking on a particular ad
For 1000 high resolution images displayed you may expect some 5-15 US$ per day revenue. Many thousands of UW photos may thus result in many times 5-15 US$ ... the more you photograph, the more you publish, the more site visitors you attract and thus the higher your revenue from advertisers using your pages as advertising platform via Google Ad Sense or similar ads.
A typical UW photographer knows how to do much more than just "simple" top level UW pictures. You are living on beaches, enjoying sun, palms, ocean or other recreational activities; hence you have thousands more photos to be published on your site. Your site may become a treasure for other potential visitors to the dive sites or locations / destinations you have visited or are living at. Hence the more precise information you publish to meet the potential need for information from the prospective traveler / tourist point of view, the more likely you find more and more visitors enjoying your UW photography oriented web site.
In addition to all a.m. you also may find challenge and pleasure in writing technical reviews around SCUBA diving equipment, diving accessories or UW photography equipment / accessories. Hence you may add new and more sections to your basic web site as you progress and feel fit to publish expert information and help.
Without starting you never learn. The basic cost are minimal compared to the basic cost of being a certified current SCUBA diver and compared to the SCUBA equipment.
If you are an UW photographer and have thousands of beautiful UW pictures laying in your drawers or hard drives on your computer - think about sharing your photos for the joy of all - and learn to earn real money that allows you to do what you love to do - to SCUBA dive at exotic destinations of your choice - amidst tropical beauties and exotic life forms.... exploring ever new sites and locations as well as new countries - all paid by experienced high end web publishing skills.