Make a part detect terrain water?

I want to make a fishing rod the only runs a function if the bobber part is touching the water from Roblox's built in Terrain system.

To elaborate, a GUI would only open if the bobber was touching the water, and if it was touching anything else, nothing would happen.

I don't currently have this part scripted, because I have no idea if terrain/part detection is even possible.

Thanks, -Mex