When trying to go down a ladder with water above it, the game does a sort of auto jump, restricting the player from going down the ladder. If you crouch long enough you can get down it but its rather difficult.
Here's a link to a visual example: https://www.youtube.com/watch?v=VW0OMweJd1s
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We're tracking this issue in MC-44560, so this ticket is being resolved and linked as a duplicate.
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