Don't know if it's a world generation glitch or a conversion glitch, but it makes it so all entities, including the player, will treat water like it's not there.
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This world was created in one of the 15w(something)a (and those were 1.13 snapshots), and I kept upgrading it snapshot to snapshot, until 1.13-pre1, which is where I found the bug.
What version was this world created in? What versions have you upgraded it to in the past?