In my boaty days, the rules for entering the UK in a small vessel at an unstaffed port or harbour were clear. You called HMRC on the VHF, moored up and were prohibited from leaving the vessel until either visited by an immigration officer or for a fixed time period. If a crew and passengers were refused entry off they would go, aboard their own vessel. The same principle applies for cross-channel ferries; there's no obligation to allow one single passenger, one single car or one single freight wagon off the vessel until entry clearance has been given. If the Border Force set up shop on the exit ramp they would leave on-board all those refused admission, who would then be the ferry company's headache.
Cameron's efforts in recruiting a Kermit economy minister to Project Fear by warning that the French would renege on a mutual advance border treaty are therefore as pointless as everything else about the man. Yes, Border Force checks can be kicked out of Calais - but you can be damn sure this doesn't mean a tsunami of illegals boarding the ferries. The ferry companies, knowing that they will be lumbered with them, will introduce their own checks. At a cost which will be recovered by higher freight and fare costs and increased travel time costs. Which will harm both French and UK trade.
The current treaty arrangements, completely seperate from EU treaty obligations, are of great bilateral benefit and despite the whining noise from Paris will not be revoked. It's just Cameron and more Project Fear to join the dodgy dossier and other miscellaneous 'Stay' lies.