Most marine photographers today have high resolution digital equipment and access to either a stable boat platform or a helicopter. This is the first step - a website gallery of impressive photos is also essential for marketing.
Unfortunately, this is a highly competitive market saturated with incredible talent. The way to make it is to first work locally, avoiding high overhead travel expenses. By specializing in a particular venue, such as organized regattas, private sport boat commissions or even real estate aerial photography, you can do quite well.
Check out The Master