If a Good Samaritan charges for the assistance the entire "Good Samaritan" status changes in accordance with both State Law and Federal Law. Remember if you are in a different State to your home State then that State Law could be different from your home State also. Basically with a fee you are now in the rescue towing business, would need a USCG license, a business operational permit from local government and of course your legal responsibilities have also changed.
Federal Statue 46 USC 2304 requires a vessel operator to respond if possible to a vessel or person in maritime trouble. Visit the Good Samaritan protocol at www.uscg.mil for full guidance on your response remembering that your first responsibility is to your own vessel and the people on board. The USCG Protocol states the parameters under which your response should be conducted. You may also call the Coast Guard Rescue Coordination Center or your Sector Command - please before you go out on the water - to get the full information.
This is one more thing you should know as a boat operator so now is the time to call before you find yourself trying to be that Good Samaritan and end up adding to the problem.