It is very Cool to wear an Elytra while we swim underwater but by doing this, the durability will go down fast and it would be such a waste to really do the cool swim thing. The problem is mentioned in SwimmingBird941's video https://www.youtube.com/watch?v=Dmd8zOUa3pc&feature=player_detailpage#t=94 .

I suggest a few solutions to this problem...
1. You Create A 'Off' Switch For Elytras

• Pressing [Sneak] button will close the elytra

2. You Can Easily Remove The Swimming Feature

• Please don't do this, this feature looks really cool

3. You Can Add A Swimming Boost When 'Swimming'

• You Dive Faster, Swim Faster etc. etc. Just Like Penguins? Hahaha
• This Will Make Burning The Durability A Bit More Worth It

That's All, Thank You For Reading