Forgery is illegal
It's illegal to forge any document and use it to obtain a benefit, I think.
I'm guessing this yacht was pretty high-end and usually chartered with a skipper.
Most charter firms only want a low-end qualification, if any (or just the ability to motor out of the berth without hitting anything). Demanding a Yachtmaster cuts your market down, I would think - most people I've met who have one either own a boat or are semi-pro yachties.