Tested at the current snapshot 14w11a, but is basically applicable to any version.
In this snapshot noticing a boat breaking at a decent travel speed, colliding with a block.
The travel distance is about 7 blocks, with a speeld of about 2 blocks p/s.
Imo boats break way to easy, and this is also buggy and not consistent, which is very annoying especially above water.
Its even worse in mp.

Why do boats actually break into items.
Wouldnt it be more logical for boats to drop as an item instead, like a minecart?
This will save to much annoyance, and will make it more consistent with minecarts and also saves a lot of unnescesarry bugfixing with boats.