Hello. I have done a check of California laws and cannot find any requirement that a marine mechanic be licensed. Nor am I aware of such a requirement in any other state. Typically a marine mechanic will have some formal schooling (e.g., Marine Mechanic Institute), but could also learn on the job as an apprentice. There is no requirement for bonding or insurance, although in my opinion it would be foolish not to be insured - bonding really protects you, not the mechanic, so it's a "nice to have". The signature on an estimate helps to protect both parties, but is not a requirement. It actually offers more protection to the mechanic, because without it you can refuse to pay (although he may have your boat).
If you would like to give me more details about your specific problem I may be able to help a bit more. Why do you think you were cheated?