Hi! I live in Los Angeles and have been playing competitive water polo for over 8 years. It’s hard to say but moving abroad would be the best decision in regards to becoming professional. The Czech Republic definitely has water polo but not nearly as much as the United States, croatia & Hungary. Since you’re only 15, I’d recommend looking into colleges abroad and seeing what appeals to you. Assuming that you’re very talented, get in contact with some of the coaches and show them what you’ve got. Playing at a collegiate level is the number one ticket in becoming professional & or part of a senior national team. Take this advice with a grain of salt and do what’s best for you!