If you hold down the left arrow key sometimes your boat will slip to the left at the beginning or end of a screen.
For the dock and lion areas it has always been the end of the screen for me. At that point you just back up (down arrow) and hit the right arrow key to bring your boat back onto the screen where you want it.
For the Explorers' Tent area it has been at the beginning of the screen. You just back into the tunnel while holding down the left arrow. You slip to the left and then let your boat proceed normally while hitting the right arrow to bring it into view.
I would like to emphasize that even if you know what you are doing and what to look for when it happens, it only happens sometimes. For me it is about 25% of the time if I am trying to do it.
Well it just might be. The explorer at the bottom of the tree (He got the point.) is the graveyard caretaker in the Haunted Mansion. Disney Imagineers love to reuse faces it seems.