It depends alot on where you're diving, and the circumstances under which you book the charter. For example, there are a lot of places in the Caribbean that cater to divers on vacation, and will provide tanks, weights, a divemaster/guide, and probably buddy you up with either a single diver or a DM. If you were to charter a boat in Massachusetts, you would be expected to provide your own gear and buddy, and there's a very good chance there wouldn't be a DM in the water at all.
Likewise, if you call a charter boat and ask them to take you out to location X, it's entirely possible that your group will be the only customers on board. If you sign up for a trip that'll happen with or without you, there's a better chance you'll get paired up with someone.
If in doubt about a boat's policy, ask first. If you get on a boat without a buddy, it's a good idea to ask if there are any divers that wouldn't mind buddying with you, and get to know the volunteers. If you wait for the DM to assign you a buddy, you might not get a buddy you'll be happy with.