Notice I said MOF.. not MOT. Ministry of Finance/Oshawa. Go back to the guy and get a bill of sale for the trailer.. minimal $250. Get a bill of sale for the boat.. as minimal as would be believed for make model and a bill of sale for the motor..for the balance. You pay tax at the licence bureau for the trailer on it's value and it's value alone. They have NOTHING to do with boats. When the boat is registered Transport Canada/Services Canada is mandated to hand over new registrations on a month end basis to the applicable provinces tax office. One day in the mail you may get a "please show where you paid tax" on the boat that you just registered. You may never get this letter.. but you might as well...it's a crap shoot. You mail them 8% on the boat bill. The motor.. NEVER.
This is all provide you bought this from a private citizen and not a dealer. If you bought it from a dealer he should have taken care of all taxes.