Judging by the marshals last weekend, you wait until a boat is past where you want it to pull in, then say 'Can you come in here, please?'. Also, you should issue random coxing instructions, like 'HOLD IT UP EIGHT!' and '2 and 4 take a stroke', so no one know's who you're talking to and everyone gets in a mess. Oh, and say 'You can't stay there' without giving any suggestion of where the boat can actually go.
Not that I'm down on marshals or anything.