I bought a SeaLand one from Defender because our yacht club had no adapters. It's a well-made product, and makes a good seal when travelling. Much better than the foam cone that some marinas use. There are three standard sizes, and the Defender site shows how to measure properly. Counting the threads per inch is a pain, but not difficult.
If you're thinking about your car, e.g. "all 2005 Chevy Malibu's have this door handle", it will not work for a couple reasons. (1) There is little liability for minor parts on boats, unlike autos, so they do not track things as closely. and (2) Boat manufacturers have much less volume than auto makers, so things change. Only a thousand "production" boats are made annually by Catalina - less than a day's run for Chevy. Many minor boat parts such as this are installed based on "whatever is in the parts bin that day". I suggest that you review the following and determine the size for your own boat.
http://www.defender.com/product.jsp?path=-1|7504|2290180|2290181&id=1987783#second_Graphic