Formal certification does indicate you have had some form of formal training. However, there is no substitute for experience. Most charter companies will ask for your sailing resume. Your experience as skipper and/or crew all counts. If you are going to a destination that calls for nights on the hook, they may ask for your experience with various anchors and the types of bottoms where you dropped the hook.
Quite often, the charter company will allow you to book a boat 5 foot or so bigger than the boats you already have experience with.
Best thing for you is to contact a local charter company, tell them you are interested in chartering a boat and ask what information they require.
Here is a sample from a company I have chartered from - http://superiorcharters.com/pdf/SCI_Resume.pdf