The Mr. C is located in San Pedro at the 22nd Street Landing. Unless, they have moved it, like so many other dive boats!!!
Their dive charters are inexpensive allowing you to bring food aboard and drinks. Or, you can have one of their yummy burgers or chicken breast sandwhiches (grilled). Usually the crew bring up a yellowtail and grill that up as well to share with everyone. They also charge for their air fills. No all divers are hard core divers and like to do three or four dives, so this is a way to keep cost down as well. Re Nitrox, I don't recall seeing Nitrox on board. In fact, I don't recall any OC/ LA county boats with Nitrox. If you want Nitrox, I would suggest the Lois Ann in San Diego.
To get to the 22nd Street Landing, take the Harbor Fwy south to Gaffy. Keep to your left. Travel all the way down Gaffy until you hit 22nd Street. Make a left. 22nd Street Landing will be the second landing on your right hand side. Pull into parking lot, drop off gear, move car to lot across the street.
I might add that the restaurant upstairs is VERY good!
Enjoy