“when I was going to get a drink in the Red Fin, I’d never thought I’d follow this random guy into the dark sea, but here we are, the undeniable champions of the bronze sea. I’m glad I joined your crew.”
“Yeah, same here, uhh… What’s your name again?”
Basically at a certain loyalty level (probably the first one you unlock) you find out the deckhand’s first name. After a certain number of wins, you also get your rival’s first name.
This is to make them feel more important, considering these are the most important NPCs outside of the story ones. It would make no sense not to know their first names.
This is peak suggestion
Learning their (deckhands and rival) first name would indeed that make them more important, maybe at the end of some side quest that you get after maxing loyalty with deckhands/defeating your rival a certain amount of time