Do you have any information about who you spoke with for this? This information is wildly inaccurate and I’d like to follow up. If you’re ever curious about what you can add to SmartThings in an officially supported manner, you can click on “+” > “Device” > “By device type” > “Water leak sensor” (in this case). This will be localized to the region associated with your Samsung Account. All Zigbee and Z-Wave Water Leak sensors in that list should execute locally.
Locally executing (internet not required) Automations on your Hub already exist (albeit in a somewhat limited fashion through Smart Lighting), did previously exist in Smart Home Monitor, and will soon exist again via the Rules API. This concern is totally understandable and something we are actively working to expand. Hopefully this information helps. I could walk you through making a locally executing Smart Lighting Automation that controls a water valve today. If that is of interest, feel free to PM me.