I want to know what type of sensor hardware I need to allow detection of moving targets (20mm - 200mm size range) in heavily murky water, where visibility range of typical camera can be limited to 0.5 meter and less.The operational range of the sensor can be not more than 10-20 meters. The goal is to detect organic bodies,estimate their size and count them using computer vision software like OpenCV. I am software guy and looking for an advice for the hardware part of this task.
As you're probably aware, the sensor solution you're looking for is pretty much a research subject in itself. There have been experiments using infrared time-of-flight cameras in murky water, and there are a few devices commercially available, but they're most certainly expensive and will require some engineering work to be adapted for a robotics application.
A relatively simpler option is to buy a set of underwater infrared cameras and implement range estimation in software, e.g. using OpenCV as you mentioned. Another alternative approach is to use sonar: there are 2D and 3D sonars commercially available, or you could try to build your own.