There are a number of options.
Bond the sensor to the hull
using silicone caulk.
Glass a small cofferdam to the inside of the hull then install the sensor in the cofferdam immersed in mineral oil
Trick is finding a good location to begin with the has no voids in the laminate and good transmission
. I’ve heard about smearing Vaseline on test locations or using a ziploc baggy full of water