There are piezo beepers that have their own oscillator built in, and there are piezo beepers that need an external pulse generator to drive them.
If it's rated for 12V, and you hook it up to DC 12V, and you get buzzing, then the system should work if you simply hook up the float switch inline.
What if you don't get buzzing by hooking up DC 12V directly? In that case your piezo unit needs an external driver circuit. A common way to make this is with a 555 timer IC. Typically you use a frequency between 1 kHz and 3 kHz.