water level sensing electronically
Too Complicated/Expensive/Unreliable

The approaches you suggest require expensive and power-consuming hardware with complex signal analysis to extract water-level information.

All you need is a few $4 float switches arranged at different depths like this:



I had suggested this approach to another similar question.

Now, life is really simple. Each of the switches corresponds to a different water level (you might want eight of them 1ft apart?). As the water level changes, more (or fewer) of the switches will be closed. 

This can be read with digital logic or a microcontroller extremely easily.

Upgrades

If you want to spend more money to improve reliability: 


You could use two (or more) independent arrays in parallel and compare the results to detect sensor failures. 
Use more expensive (higher quality) float switches
Increase the number of float switches