As you progress, you can complete achievements to earn special themed ducks, such as gladiator-duckie, aviator-duckie, dino-duckie, and several others. A "Choose your duck" option lets you pick a themed duck to use as your avatar that will show up on the map below the level currently you're working on. There isn't too much more to the themed ducks than the costumes and funny descriptions, but I think it's a neat way to track achievements.
Now for what I don't like
It's true that a lot of games these days are using various types of freemium models. comes to mind, which makes you wait for getting a car serviced and also to have a car delivered that you paid for with in game cash. This model is annoying, but like I wrote in my review, once you own a few cars, it's pretty easy to switch cars and keep playing the game. It's not ideal, but it's workable.
With Where's My Water 2 you don't have a workaround. When your energy meter runs out (after about seven played levels), the game forces you to either connect your Facebook account for one free refill, or -- after that one-time refill -- bother your Facebook friends to "give you energy" to continue playing. If your friend doesn't own the game, he will be asked to download it to help you out. One small plus is that it asks your friend directly and doesn't put the request on anyone's Facebook News Feed. Your other option is to buy a single refill for 99 cents.
Unfortunately, if you want the game to remain free, that means you'll have to wait, and I saw wait times of over an hour after a few plays. I e-mailed the developers of the game to find out what determines the amount of wait time, and will add more information once I hear back. To make matters worse, when the time is up, you only receive a small amount of energy, letting you play a couple of levels before you have to wait again. Your only option at this point is to put the game aside for a longer period of time to replenish your energy to full so you can at least play through a few levels before having to wait again. If you're an adult, you might set the game aside for a while, annoyed you have to stop playing. But if the game is for your child, I can only imagine the frustration for both the kid and his parents. This model is totally unacceptable and I would much rather pay a premium price (such as $4.99 or even more) to buy the full game than to have to jump through these hurdles.