Make sure that Airmar, and the yard manager get you the correct Snell's equations.
The refractive index is important. Air = 1.00; Water = 1.33; Polyurethane = 1.34. At the ultrasonic boundary of the xducer and water, that delta of (n) is critical. Note that water and poly... is about the same. That means not much will happen at that interface. Note also the big difference from water to air, that is significant, setting up a large reflected wave.
It's possible air entrapment may make an issue, in some cases. That's why I suggest painting; that should have a leveling effect.
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