Patrick
I am the developer behind one of the Underwater Laser Scanning systems above. My company 2G Robotics developed the ULS-100
http://www.2grobotics.com/products/uls-100-underwater-laser-scanner/As you indicated above some sonar systems can achieve very high accuracy at high distances. The challenge with sonar is not accuracy it is precision. As an acoustic pulse travels through the water it expands from the source to the point that the acoustic energy contacts the target surface. Depending on the system and the travel distance from the acoustic head to the target surface, the acoustic footprint (area that acoustic energy is reflected back to the sensor over) can range from 10cm for short range high frequency heads to meters for long range low frequency heads. Since the footprint of the sonar is over a large area a lot of detail is not captured by the sensor. This is similar to a very low resolution image that looks all blocky. In the image you can make out the general shape and know where things are but small features get washed out.
Laser Scanners do have a small footprint but it is significantly less than sonar. The drawback to lasers over sonar is the need to be close to the object being scanned particularly in turbid water (low viability water).
The ULS-100 is a relatively close range system and can measure objects at 1mm precision from a 1m offset distance. Using common techniques for assessing the status of underwater assets such as sonar and video require significant assumptions to be made. The ULS-100 can provide quantitative engineering data at high precision for damage assessment.
Below is a scan of a fan blade that was scanned underwater and a picture of a section of mooring line rope that was scanned underwater.
I hope this helps you understand the benefits and drawbacks of laser systems vs sonar systems.
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