I've got a Moen Flo water sensor I really love. It was something like $25 with no monthly fee, has a surprisingly good app (also measures temp + humidity) and can even be paired with a valve that automatically turns off the water in the case of a detected leak.

What I like best about it is it can not only set off a siren and notify me in the app, it'll also send a text message and even call my phone which is huge because if I have a water leak I want to know no matter what.