Placing a boat in a narrow canal or on a water block adjacent to a solid block (which has air above it) causes the boat to move in a jittery manner when ridden. Upon disembarking, the player teleports back to the exact place where they got in the boat; the boat then becomes unresponsive to anything other than water currents or gravity. It can't be pushed, can't be ridden, can't be ignited for more than a barely visible flash, and can't be destroyed by hand, sword, explosions, lava or arrows (they go through it and become invisible, but they can be picked up from the wall behind the boat). The only way to destroy it, AFAIK, is dropping it into the Void.
Boats placed farther from the coast don't behave like this.