Swimming pool depth sensing
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.