Is it possible to detect when a part hits water terrain?

Hey there TeaWithMee! Sorry to be the bearer of bad news but, the only way to detect when a part hits water terrain is with an invisible part (like you said you did). You probably already know this I'm going to put it anyway just incase someone else doesn't, preferably with CanCollide disabled and leaving CanTouch enabled (this is important if CanTouch isn't enabled it won't read that it's touching).

Here's a helpful link that includes all the functions and events (plus everything else) of terrain. https://developer.roblox.com/en-us/api-reference/class/Terrain

So in short it says "Determines if the part will trigger Touched/TouchEnded events on other BaseParts with TouchTransmitters" to continue on this it also says "Fired when a part comes in contact with another part".