It is available to use some types of fields, like acoustic or electromagnetic to detect it. The main idea consists of waves scattering on plastic bodies. If there is an appropriate boundary between two media (plastic and water) you can pick up a frequency of the acoustic wawes, which is able to scatter on the plastic bodies and your detectors will be available to receive the scattering waves. Why it is an acoustic wave? There is a problem with interaction electromagnetic wawes with the water and it is a more sophisticated issue.
If it is expected to appear only big bodies that can be sought by a video camera - you can use it and this system might cost less.