Clients have absolute control over boats ridden or once ridden by players, and ignore positioning updates from the server. Discrepancies in the client and server logic result in minimal desynchronizations, but they add up over time. In addition, boats continue to ignore the server even after their rider as exited. This can lead to situations such as a boat breaking on a shore 20 blocks ahead of the client's perceived position of the boat because it has already reached the shore serverside. A good way to test this glitch would be to create a superflat water world covered with lilypads (preset = "3;minecraft:bedrock,2*minecraft:water,minecraft:waterlily;24;oceanmonument,biome_1"). Ride a boat around at a decent speed without breaking it. Eventually, lilypads will begin to break in areas the clientside boat is not even near (clientside bounding box can be seen with f3+b). That marks the position of the true / serverside boat.

The client's boats MUST listen to position packets from the server. To prevent extreme rubberbanding, maybe even only listen to absolute position updates - which is done to a limited extent in 1.8. Whatever the solution, we need boats to not break randomly mid-voyage. We want to have peace of mind using only 1 or 2 boats a voyage as opposed to 6 or 7. Testing this in a survival world by attempting to navigate a river would be the best test case.