Maybe I missed something. What are you going to use the epoxy for? If its for setting a transduce (TD) where are you going to mount it? Understand the TD sends out the sound and also receives the reflected signal back when the sound wave hits an object. Water is a great medium for transmitting sound . Air sucks and will attenuate the sound so that your reflection will be near Zero. Another thing you need to understand is the angle of incident is equal to the angle of reflection. So if the sound impacts something at say 45 degrees the reflected sound will be reflected at 45 degrees. That why you want to make the sound enter the water perpendicular to the surface of the water. This give you maximum reflection back to your unit. The sound is also scattered because surfaces are not perfect so the sound reflects in all directions but most is at the angle of incidence. You also have beam spread going on think of that as a flashlight and the area of the sound increases the further your away from the light source.The main strength is at the center so that s where we focus on.

Sound travels though different objects at different speeds. Starting from the transducer each interface you have will reduce the sound some what. The greater the difference in the materials ability to transmit sound the more sound you will loose. As air is a bad transmitter, thats why we cant hear each other from long distances and wales can hear each other for very long distances. Select materials with little air in their makeup.Thats also the reason you don't want air in the epoxy or what ever medium your going to use. You lose sound. Plastics, glass, metal, etc have little air in them while wood, concrete and bricks have lots of air in them. I would try using a silicon first as this would allow you to remove the TD should you decide you wanted to do something else. you could also try the set with Vaseline or petroleum Jelly as a temporary set up. It will let you couple to the surface and see how it will all work. Hope I didn't confuse or make this more difficult than necessary. If you have question you can PM me.