Gearbest has the Xiaomi Aqara Smart Water Sensor for $13.86. Use coupon code GB11.11ConE
for 17% off to drop the price to $11.50. Free shipping takes about a month to arrive.
This is a Zigbee sensor I believe. Works with Samsung Smartthings using a custom device handler. It can be tricky to pair to the hub but once set up, it works great. Cheaper than Samsung's own water sensors.
Pairing instructions to SmartThings can be found here: https://community.smartthings.com...r/97584/21