This can happen if there is heavy magnetic interference on your location. Water picks up electrostatic radiation. Therefore, electronic devices known for electromagnetic radiation should be kept away from the water to be recorded in. (switching power supplies, fluorescent lamps, CRT displays or TV sets, …)
The ASF has a high-gain pre-amplifier and therefore will also amplify any electromagnetic signals the piezo sensor detects.
But this can easily be resolved by establishing an electrical connection between the chassis of your recorder and the water that touches your hydrophone.
The best way to achieve that is to use a wire that is connected to pin 1 of a free XLR input and held into the water near the Hydrophone.
We have a grounding cable available in our portfolio: AHK-ERD10.
But any electric connection between recorder ground and water will do and should get you rid of any unwanted noises you might experience.
ATTENTION: Never do so when recording from a boat and your mixer / recorder is connected to
the electric ground of the vessel. If you are experiencing inducted mains hum ALWAYS
disconnect the ground of your mixer / recorder from the power supply ground (e.g. use battery
power instead) before you connect the audio ground with the water.