So this is a minor bug that I just found while exploring the new beta version of Minecraft Bedrock Edition and that is the weird arm animation when swimming underwater. Just go underwater and start swimming and you'll see it for yourself. As you see, there are only 3 frames of animation as well as the arms being dis-proportioned. It's really minor, but a bug nonetheless! Hopefully, you fix it soon, awesome team!
Thank you for your report!
We're actually already tracking this issue under MCPE-66475.
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