Yes and No
According to NC statutory law, unless a property is fenced or posted 'no trespassing' in, there is no law prohibiting you from entering a property. There may be local laws or ordinances that DO prohibit this however. If an owner or person in legal control of the property (i.e. tenant, family of owner) requests you to leave, if you refuse or go back at a later date, you can be charged with trespassing. So your friend is technically correct... Put it into practice and you'll eventually make some dock owner upset. I'm not sure about GA, FL, etc. If you must use a private dock, at least contact the owner first.