Elytra deactivates upon hitting water. Upon coming in contact in water mid-flight with an elytra, one will stop their flying even if going rather quickly through it. This would not normally be an issue if firework boosts were not a thing. Launching from the water is convenient if not entertaining sometimes when the user carries many fireworks.
Steps to reproduce
Fly elytra through water
Come in contact with water

[helper] Jack McKalling stated this on the java issue MC-97190 : 

"Understandably it was marked as intended that the gliding continues when you get into water (so you can glide through)."

this is a parity issue, as it has been marked as intended to work in java and not to work on bedrock.