Piezo to detect low frequency, low strain
To detect low frequencies with a piezo requires an extremely high impedance amplifier, for example if the element is 10nF you'd need something of the order of 3-10M ohms. 

Try a low bias current CMOS op-amp with (say) 4.7M load resistor (parallel to the piezo).  Choose one with bias current &lt;&lt; 1nA ( easy- there are parts with &lt;&lt; 1pA). 

Your circuit may not be good enough for practical use if the resistance gets much higher than a few M - condensation from the cold water, for example would have to be avoided even at the suggested resistance level.