Well, I guess there really isn't a "standard" for that. I guess the best way to explain it is like a tip out situation.
If you've ever worked as a waiter/waitress/etc. you "tip out" at the end of the night to your support staff. Waiters/waitresses tip out the bartender, bus boy, and sometimes even the kitchen. Bartenders tip out the barback/busboy and sometimes the kitchen. In this situation, whatever you give to the DM (assuming you hand your tips to the DM) he tips out the captain and the rest of the crew as applicable. In my crew's case, they consider themselves equals and therefore split whatever is given to either of them equally...except in cases as mentioned above where the customer gives them tips individually and indicates that. IMO, Captain's don't get enough credit. The divemaster is the one showing you a good time on the boat and underwater, but the captain is essential to the mix as well. Especially when it is a team that works together everyday and knows each others work habits, etc.
About a year or so ago, Raul (my previous right arm DM/Instructor extraordinaire who has moved to NYC) rolled off the boat and forgot his reel and buoy. Of course he didn't realize it until well into the dive. No worries, Mago noticed he had forgotten it right after he descended...timed the dive, and lowered it down to him about 40 minutes into the dive with line from the boat so he would be able to deploy it when needed. He literally lowered it down right in front of Raul's face
He's done the same type of thing when I've been working with a student. I left my slate on the boat, he knows I use it with students. While kneeled in the sand doing open water skills, he lowered my slate down to me. I don't know any other captains that are that intuitive and on top of things
He's awesome