I echo Duke's observation wondering about how a lake can be 50 feet deep a mere 10 feet from the shore. Sure, it's not impossible, but really? It all seems merely a device to allow Jaime to survive the battle to be reunited with Tyrion, deliver the news of Joffrey's killer to Cersei, or both. The Dothraki, you will recall from season 1, aren't exactly known for taking prisoners.

I wouldn't be surprised if, next episode, during the post battle cleanup & burning of the bodies, an unarmored Jaime pulls himself onto shore and collapses, and right as a dothraki rider is closing in for the kill, Tyrion stops it, and voila, there's your reunion. His interactions with Dany, given that he killed her father, should be particularly interesting.