Even if this were in HSM (which I don't think makes sense), you'd still have to remember to update the app every time you add a new water sensor. HSM (and every other app) creates subscriptions to device events when you hit Done. So if you add a new water sensor, there won't be a subscription to it unless and until you open HSM and hit Done.
Hmmm, with RM you'd have to open the trigger, click on the water sensors, select the new one, and then hit Save, Done, Done... Meanwhile, your use case is not an obvious one nor universal. Alert for each sensor when it goes dry, or for all of them becoming dry? It gets hair on it pretty quickly! That's why RM exists, so you can customize an automation to your use case.