I don't think AT&T has a waterproof phone yet. I know that both Verizon and Sprint have one, but I haven't read anything about AT&T unveiling one. Another option is to get waterproof coating for your cell phone. I'm not sure how effective the product is as I have never tried it.
Waterproof cell phones are available, but unfortunately the only carriers for them are Sprint and Verizon. Companies like Motorola are working on a waterproof phone for AT&T, but consumers may not see them until next year.
Yes the Motorola Tundra is a water proof phone and will take a certain amount of water. No phone is going to be able to go deep underwater but rugged phones are able to take a certain amount of liquid and be unaffected the tundra will take this type of abuse and is available from AT&T.
There is not a waterproof AT&T phone. The only phone for AT&T that is made specifically to be durable is the rugby, but even that phone is not waterproof. All of their phones have water indicators.