Actually, they have been cracking down at the border. If your motor boat or sailboat is more than four and a half meters long (14'-6"), you will need to get a Temporary Vessel Import Permit in order to cross with your boat in tow. They will also ask for the original and a copy of the boat registration, your passport, driver’s license and title to your vehicle. Unfortunately, you cannot obtain the permit in person at the Lukeville crossing. The easiest way to obtain the permit is to apply online, so I hear. I am fortunate as my RIB craft is documented as 14 feet, even though it really physically measures longer, so they haven't hassled me so far.