A sailing certification, though helpful, is not required. Most charter companies will have you fill out an online sailing resume, and will respond to let you know what size of boat they recommend for your charter. Even with a certification, charter companies want to see that you know what you are doing. They may have a staff member go with you on a brief sail out of the harbor to make sure you are comfortable with the boat.
If you are not comfortable with the boat, need a refresher, or would just like a few hours with a mentor on a bigger boat than you normally sail, the charter companies will gladly send a friendly skipper out for an orientation sail with you. If desired, you may ask your charter company about hiring a skipper to spend the first day or two sailing with you and your crew. This can be a great way to get some valuable training or a refresher on rusty skills.