Question: **OBJECTIVE:** A device that floats on the water's surface and measures the depth. If any changes occur in the depth, it sends a report to a remote receiver.

**MY RESEARCH:** I have considered the following sensors but they don't seem to be effective:

1. IR won't work because water absorbs red light.
2. Ultrasonic: I can't find one that works underwater.
3. I also read a thread: [Underwater distance measurement/Sensing](https://electronics.stackexchange.com/questions/55804/underwater-distance-measurement-sensing) however, I don't know if I can implement that.

**QUESTION:**

What sensor can I use for this purpose?

Answer: 
A pretty standard fluid level sensor (optical, capacitive, ultrasonic, IR) will mount to a fixed point and look down at the fluid.

If you must have the sensor float, consider measuring distance to some fixed point above the water (since your floating platform will be on the water, it is at water level).

Laser rangefinders should work, although depending on the range and precision you need the result may end up large. You can also make up for it by slower sampling and a proper filter.

The real simple solution however would be to drop an anchor. If the tension slacks or angle of the anchor line goes beyond some point, your level has dropped. If you used a torsion spring to maintain tension you could even count it as how much this spring has turned.
Does it have to float? A pressure sensor on the bottom would do the job and be simpler and more reliable, even if you need to run a tether to a surface antenna, whether that antenna is fixed or floats on a buoy.