How do you approach your boss about asking for a raise?

Absolutely!
There is no "Easy" way to approach this topic. You want to find a time to talk to the boss, when he is not rushed. You might tell him that you need a few minutes of his time to discuss something, and hopefully he will then bring you into the office where the two of you can talk privately. You will want to point out the added responsibilities, the fact that you have more responsibility for the overall success of the marina, the oversight (and training?) of other workers, scheduling (?), etc. Have a figure in mind, in case he asks you. Do you have any idea what the previous, or other supervisors are paid? Sometimes a supervisor only makes a little more than the other people, sometimes a lot. I'd recommend trying for at least a dollar an hour. (I've known some companies that pay only 25 cents more). If you avoid this conversation, he will take advantage of you. He should have included a raise when he gave you the position. Of course, it is possible that he did, but failed to mention it. It might be on your next paycheck. But I would not wait that long. Have the talk. You will need to learn to be able to do this throughout your career, now is a good time to start!

Good luck!

1. Share your goals and ask for feedback.
2. Proactively communicate wins.
3. Demonstrate your accomplishments and added value.
4. Focus on why you deserve it (not why you need it).
5. Practice your pitch and anticipate questions.
6. Do your research.
7. Talk about the future.
8. Be prepared to hear no.

https://www.forbes.com/sites/elanagross/2016/06/27/8-managers-share-the-best-way-to-ask-for-a-raise-and-get-it/4fd83ba74ff2