A private individual who allows someone to use their boat (or car, truck, etc) is assuming the liability for anything the operator does. Your insurance will be the primary in an accident and any insurance they have will only come into effect when the owners is exhausted. So I would never let someone outside family use my boat without my presence.
If you were going to start a business to rent boats set it up as a corporation and buy insurance to protect the assets of the business. Do not take on the personal liability for someone you don't know.
Here is a company in Tennessee that rents wakeboard boats. http://norriswake.com/reserve.php