The bug
Swimming can be activated as long as the player's feet are touching water, instead of the eyes. Sprinting across shallow water will interrupt you and start swimming, even in the smallest possible water level:
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Demonstrated in this brief video:
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Expected behavior
The player should only begin swimming when sprinting and the eyes are underwater.
Thank you for your report!
However, this issue has been closed as Invalid.
Going forward we're going to be tracking parity issues and feature requests on the Minecraft Feedback Site, and we'll be tracking bugs here at bugs.mojang.com.
We're already working on achieving parity across versions where possible, and we're very keen to hear your requests or suggestions, so we'd encourage you to make a post at https://feedback.minecraft.net, or upvote an existing topic there.
If you're not sure whether your issue is a parity request or a bug, hopefully this article will help explain further: Parity Request Guide
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