When a (Deep) Warm Ocean or (Deep) Frozen Ocean biome is close to a temperate landmass, the world generation create jaggy, repeating transitions, which tend to follow a North-South or East-West axis.

Notice how the water color changes in the provided screenshots, and look at the biome map given by Amidst. (The world seed is -7748547350291891684, and the screenshots below are at -1282 1772 and -2560 1530)

Note:
This change seems to be related to a 5x slowdown between 18w15a and 18w16a when generating the biome map with Amidst, which use the GenLayer class of the Minecraft .jar to calculate biome data. It is likely that this slowdown is also present in-game when generating chunks.

Note bis: When diffing the MCP-decompiled code for 18w15a and 18w16a, I found this change, which may be the culprit: https://pastebin.com/fQfT5BUW