Yesterday, I was in the process of building a underwater mic. What I'm doing is taking a mid range driver (A type of speaker) soldiering on audio wires, then dipping the final product in plasti dip while making sure that no part is exposed to water. I did the first dip and it covered most of the speaker, unfortunately the dip missed a couple screw holes, so later today or tomorrow I'm going to cover the rest.
My first test is going to be in a bathtub. Some time this week I'm going to call up my local health club and see if I can use their swimming pool to test distance pickup ranges.
My in the bathtub tests, I'm also going to submerge most of a plastic bag with a speaker attached. This will test if a hydrophone can pick up any sounds when being transmitted from air to water. The third design is taking a plastic cone attached to a speaker being held out of the water. The sound will travel down the cone and vibrate the water, this will test if the sound outside of water can be recorded.
Ahhh, what a weekend.