Which is to say, what did marines do in the ships outside of combat? How were they treated by the sailors - were they "one of the men," or an exclusive group?
I've written about this a bit before; you can find an older answer here: https://www.reddit.com/r/AskHistorians/comments/3oijlw/what_was_the_relation_between_sailors_and_marines/
and Marines and their use after mutinies here:
Let me know if this is what your'e wondering, or if I can answer more questions for you.