The Achievements Category API returns this achievement as part of the Events category:
/data/wow/achievement-category/15274:
“achievements”:
“name”: “Crashin’ Splashin’”,
“id”: 13927
But it is not returned by the individual achievement API:
/data/wow/achievement/13927:
404
Edit: also missing: 13924, 13925 (from a different category)