Large ships have a ballast tank system to compensate for different loads, so that the ship is as balanced as possible in water. Water sucked in for ship balancing in one ocean is prohibited to be mixed with other ocean water due to regulations. The IMO (International Marine Organization) states that such systems would kill microbes and small animals, which are sucked in with sea water. The amount of water sucked in and recirculated must be reliably measured.
A big problem is space limitations inside the ship - pipelines of up to 500 mm are typically installed at the vessel wall, making an in-line flowmeters installation impossible. Inlet and outlet straight runs are typically missing, which makes the use of ultrasound devices very difficult in addition to the onboard sonar that may disturb an ultrasonic measured signal.