As others have said, yeah, the belt. But as you said, it should get tougher not lock up. Well, the water entering the engine bay is still a problem. Do you have the undertray? If not that helps a lot to protect. I know that if I hit a puddle with good power steering the water holds the wheel and makes it tough to turn the steering wheel. So, if that happens to you and add the loss of power steering at the belt, then it might appear as a lock-up. I hope it is as simple as that for ya so there isn't a bigger problem.
Good Luck.