Underwater Ultrasonic Thickness Gauage

Hello everyone. I am working on a project which requires me to come up with a way for an ROV to measure the thickness of a material(most likely Plexiglas) floating on the surface of the water. After digging around a bit I have found Ultrasonic Thickness Gauging to be the most prominent and plausible way of doing this. However I am having a huge amount of trouble figuring out what sensors I should use to achieve this. Most sources seem to agree that waterproofing aside I can't use a transducers designed for use in air underwater. However does this apply if the sensor is in direct contact with another material anyway? Also does anyone have any idea what transducer/sensor I could use for this project? All the ones I find are either not designed for use closer than a few inches, or are not designed for use underwater. Any advice would be appreciated.

As I understand it, thickness gauging works with the transmitter/receiver attached to one surface of the test item. This looks inapplicable to floating objects. Instead you should track echoes from the front and back of the object, and eventually distinguish these from echoes from the water surface itself.

I'd start with a underwater probe (Cygnus...) and a scope, to find out more about the echoes in your scenario.