When you combine canteens in a crafting grid you get canteens with 'interesting' values.

Example: Two 1/3 full Dirty Canteens (toughasnails:canteen:9) in a crafting grid creates a "Clean Water Canteen" (toughasnails:canteen:15)

I believe this is due to the canteen inheriting the vanilla minecraft property of normal tools to be able to combine 2 of the same type of item create a single repaired item.

You could add: this.setNoRepair(); to the public ItemCanteen() section of the ItemCanteen.java file.

The idea I had was to be able to combine canteens in crafting so that a player can "pour" like-water canteens together to fill them a single canteen ( a 2/3 canteen + 1/3 canteen = Full canteen + empty canteen)
However with the properties of repairing like normal tools; this is a problem

This was fixed in ed42f34

Perhaps water pouring may be added in the future, although given how few uses canteens have to begin with I'm not sure it's really worthwhile.