Honestly, I have never heard anything like that. I developed tinnitus from a range day where I bought "fitted" plugs but these were fitted not molded. Small, medium, and large is not the same as a custom-molded plug. Foam plugs are effective at blocking out noise. But I have never heard anyone say that molded ear plugs were less effective than foam. And I have done a lot of research on the subject.
Whenever I hear shooters talk about hydration, my bs-meter goes into overdrive. I have heard hydration being blamed for pulled shots, poor vision, and every other malady you can think of. A red herring most of the time unless you are talking true dehydration.
As to the ear canal... most water is stored in the fat layer of your skin. Once you get to the inner ear, the fat layer is very thin. I have a hard time believing hydration levels will substantially change inner ear dimensions... unless you have some academic studies you'd like to share with us. I would be curious to read them.
I am a Certified Industrial Hygienist. I have been dealing with noise, hearing, and hearing protection for almost 30 years, and was interested in and did some studying before that (shooting and flying mainly). I actually got Howard Leight into the shooting world. I harassed Howard himself every time I could.
I have talked to all the mainstream hearing protection companies.
The ear canal is not static. It gets bigger and smaller for various reasons. Hydration level is one, according the hearing protection experts I have talked with. Also a weight change can cause a change in size. And historically, molded plugs change their performance over time, even day to day. The other issue, if the molding it not under constant pressure, you will not get a tight fit. A foam plug is constantly trying to expand a bit, keeping the tissue compressed.
I am interested in how they are getting 37 dB noise reduction. However, I do notice, I did not notice a claim of 37 NRR (Noised Reduction Rating). NRR is the industry standard for rating hearing protection. However, one thing to realize, it is not independently tested. The company does the testing and then makes its claim. Years ago, a certain very well know brand was found have been gaming the NRR testing, and coming up with an NRR several dB higher than "regular" testing would get.
Dehydration can be the cause for many issues. When you feel thirsty, you are about 10% low on water. And from flying fighters, if you feel thirsty, there is a significant decrease in your tolerance to G forces. From a heat stress standpoint, even small amounts of dehydration can cause symptoms to start appearing. Could they cause a miss? I think so.
