So I don't know if this is a bug or not. But you as the player character can swim. When you have elytra on .
Step by step :
Stand on a pilar adliest 6 block high
have elytra on
be next to the ocean
fly with elytra into water and there you go.
Mojang please note that the swimming animation looks great and that many players would like to have it, not the walk in the water animation. Please think about that. My Username is FrankoXD if you need it for something .