You should park your boat on the beach in Myrtle and try to keep folks off it. A sign like you suggest would draw unwanted attention and have poor results. Last year I was able for the first time to get law enforcement to help protect my boat while on the beach. They actually were happy to throw people off it and arrested several under age drinkers as a result.
I have made very minimal expenditures to convert my boat. In fact I have less than 3k in the whole deal, including the initial purchase of the 18. Still have a solid standard 18 mast and full set of sails and rigging. I could convert it back at anytime. I couldn't begin to buy a F18 for that little money.