How to calmly and confidently respond to classmates mocking a Master's degree in water-related subjects in the UK?

I'm sorry to hear that you had an unpleasant experience with your classmate. It can be frustrating when someone makes negative comments about your choices or circumstances. It's important to remember that everyone has their own opinions and perspectives. It's best to try not to let their comments affect you too much and focus on your own goals and aspirations. If you feel the need to address the situation, you can calmly express your feelings or simply choose to ignore their remarks. Remember, it's your life and you have the right to make your own decisions.