hydrolocking is when water enters the intake, and to make it short, the water locks up the engine. To avoid this with a CAI, it is HIGHLY recommended to spend the $50 and get an AEM bypass valve, which lets air in, keeps water out.
i drove around in rainy FL with just the CAI for about 7 months and not one problem. But when it rained, BOY did i drive carefully. I finally just got the AEM bypass valve. I feel a lot safer now. I'm not about to go try it out, i just hope it does it's job when it needs to.
Get the PR CAI,, it's definately worth it, and if you are scared or don't like the cai, you can always make it wai. It's 2-in-1