When holding A to float above the water, you are able to crouch. Doing this makes you quickly sink to the bottom of the body of water. You cannot uncrouch, swim, or jump when this happens, so you are likely to drown.
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Thank you for your report!
However, this issue is a Duplicate of MCPE-32478.
If you have additional information, please add it to that report.
Please search before reporting, as it's likely that one exists already.
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⚠️ Issues that concern Minecraft (Xbox One Bedrock version) should be posted on the Minecraft (Bedrock codebase) tracker. I moved your ticket to the correct section.