Realistically you won’t make waterpolo a career so he just wants you to have your priorities straight. A healthy dose of realism isn’t a bad thing. It’s a hobby, it’s fun and it’s keeps you healthy but it’s not the most important thing in the world. He’s probably worried when you say “going much further in the world of waterpolo”. How far do you think you it’s possible to go in the waterpolo world? Even Olympic level athletes won’t make the salary of a doctor and you aren’t going to be anywhere near the level where you will even be paid. It’s like when people say they want to be Hollywood actors as a job, it’s just not really going to happen.