Dry out all your membranes

Peter : That's a good question. Where does a sore throat go away? What do you do to get rid of a sore throat?
Mary: You gargle with salt water.

Peter: What are the odds of having a sore throat when you get out of the ocean after swimming in warm, tropical, salt water? About zero. If you actually went swimming there with a really bad sore throat, you might make your cold worse eventually. But if you go swimming in the ocean, especially if you go where there are some good-sized waves, chances are that all the salt water around will dry out all your membranes.

Hi, could anyone help me understand the above words? I don't understand the logic of Peter.

1. Peter mentions swimming in a warm, tropical salt water, so you won't have a sore throat, but why if you swim there, your cold would worse? It sounds contradictionary.

2. What does menbranes refer to? Does it refer to the mouth membranes? How is that relevant to sore throat?

What is the logic within the three sentences in blue? Do you possibly explain? Thanks!