I don't know if this is an intentional feature or not, but when swimming horizontally at the surface of water, you don't get oxygen anymore. I looked at the recent patch notes and they didn't mention anything regarding this.
How to recreate:
1. Sprint underwater to go horizontal.
2. Swim up to the surface, but don't leave the water or your horizontal state. You should normally get oxygen, but you don't anymore.
Linked issues
duplicates 1
Comments 2
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We're tracking this issue in MCPE-170969, so this ticket is being resolved and linked as a duplicate.
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Agreed, it is annoying. You are swimming on the surface definitely, but your air bubbles decrease until you start drowning.