When you go in the water your jump button is replaced with the same buttons as creative fly mode (useless but that's not the bug). If you hit the fly down button, you immediately sink to the bottom and are unable to swim up or jump. You are stuck to the ground. You can only swim again if you exit the world and reenter (or go into fly mode in creative).
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-32478.
Your additional information and observations are very useful, so if you can add them to the comments section of the parent issue it would be appreciated.
If you have not, you might like to make use of the search feature before making a new ticket, as it's likely that the issue has been reported already.
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