Water Level Sensor Circuit
Actually, if you just need discrete high/low levels, float switches would work fine, are super reliable, and not expensive.  Look for a sump pump type float switch.  Basically a mercury switch in a sealed floating ball.

If you need actual level, one of the best systems is a 'bubbler', which uses a controlled air pressure to literally blow bubbles out of a tube, which has the endpoint at the bottom of the liquid stack.  The level is measured by monitoring the air pressure and air flow supplied to the bubbler tube.  Bubblers are good as they are inherently self cleaning, and give instant indication if there is a severe clog or fault in the system.

Another option for actual level would be ultrasonic.  Ultrasonic rangers aren't expensive, and using ceramic transducers, the corrosion problem is eliminated.