In 1969 we took a 20' Bertram power boat to an Island 8 miles off our coast - Kangaroo Island - to use it for commercial Abalone diving, and as we needed to tow the boat around the Island (it is a large island with a good network of roads) we launched the boat with the trailer strapped tightly up under the hull and puttered across "Backstairs passage" (the straights separating the island from the mainland) to the island. We did this on purpose to save quite a large freight cost for the boat and trailer and not as some forgetful accident. The Bertram obviously wouldn’t get up on plane but we did travel at about 10 knots although the whole thing would “wallow” quite alarmingly when we throttled back and/or made any manoeuvres other than straight ahead under power. If some one left their trailer attached to their boat by accident, they would know it immediately the pair were afloat if not for the atrocious boat handling then for the banging of the trailer on the hull (unless it was strapped purposely up tight to the hull)
Mind you it was very "uncomfortable" and if given the choice, I would never do it again. We drove the boat back by sea a few months later but took the trailer behind the car on the ferry/frieghter and paid the frieght costs.