Absolutely impossible to continuously bob up and down in water without switching where your fingers are touching on the screen. Also, the short time window between the button switches when resurfacing triggers trident throwing animation.
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Thank you for your report!
We're resolving and linking this ticket forward as a duplicate of MCPE-179689, as that ticket contains more detailed information and/or has already been triaged by Mojang.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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