Boat

Boats are rideable objects placed on Water used to move faster through big watermasses.

Boat Riding
Quite differently from the Minecart, boats moves accordingly to the player's will. The boat also moves significantly faster than a player on water but is slower than a Minecart. While some movement bugs are still present, it's possible to steer using the WASD keys. Boats also have movement relative to the camera, so it's possible to maneuver them with the mouse. Boat changes direction if in contact with, a player, another boat or a mob. If you ride a boat and crash into another boat your boat will get destroyed and the other one will just bump away.

Boats can be maneuvered in snow and ice, but very slowly and you are confined to the same square.

Boats cannot be used on Lava.

Like Minecarts and roller coasters boats can be used to make water slides.

Video that shows how boats works (notice how the player uses waterslides for speed, and how it's possible to elevate boats by waterfalls).

Patch 1.0.6_01 photo addendum
Boats are no longer destroyed on contact with solid ground immediately. You must be going at high speed (you can see water particles spraying up) but still seem to break on contact with mobs, so they should be avoided.