I saw this as a result of another bug, where air voids form under water (you see here this is an old segment of the server but this still occurs in new segments). The rendering of these areas seems to get cut differently along a chunk (region?) boundry.
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Ezekiel
Duplicate of MC-36395 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.