What I expected to happen was...:
The Sculk Sensor would detect the splashing sound played when the bobber sinks in the water. 

What actually happened was...:
It didn't detect the splashing sound.

Steps to Reproduce:

1. Place at least two water sources next to each other in a contained area 
2. Place a lily pad on top of the one of the water sources (to prevent the smaller splashing sounds that the Sculk Sensor can actually detect)
3. Place a Sculk Sensor nearby 
4. Cast the fishing rod into the water sources, making sure the bobber ends up underneath of the lily pad 
5. Wait for the bobber to sink when you're able to catch a fish