You have (at least) two options depending on how much you plan to use Alexa to make announcements. If few, then I'd recommend creating a virtual switch that you turn on when water is detected. Create an Alexa routine to say what you want when that switch is turned on. This gets around the limitation that sensors don't appear to Alexa directly.
If you plan on using Alexa to say a bunch of different things, then look in Echo Speaks with Actions
