Waterfall Basin has the narrowest confirmed footprint of any form in this dictionary: the corpus shows it named exactly twice, both on facing pages of the same Vol. 3 fight (p.31 and p.32), and nowhere else across the twenty-three volumes searched. That's consistent with the scene's own framing — Tanjiro's internal narration explicitly states he's cycling through "technique after technique" because he can't parry the attack head-on, and Waterfall Basin is one entry in that improvised, once-only chain rather than a technique he returns to as a signature move.
There's a small but genuine OCR quirk worth flagging in the sourcing for this entry: the first occurrence (Vol. 3 p.31) is scanned in the corpus as 滝霊 ("waterfall spirit"), which is almost certainly a scanning misread of 滝壷 — the two share a component and are easy to confuse at low resolution — while the second occurrence (Vol. 3 p.32, one page later, same fight) is captured correctly as 滝壺. Both panels' VIZ text reads identically, "WATERFALL BASIN!", confirming the intended technique is the same in both cases despite the raw Japanese OCR disagreeing with itself a page apart.
"Basin" is a fitting one-word translation choice for 壷 (tsubo) in this context: the character's more common translation, "pot" or "jar," would read strangely for a sword technique, but "basin" preserves the sense of a hollow the water has carved out over time — which maps well onto the visual of the technique itself, a strike heavy and sustained enough to crack straight through a stone wall in the panel where it's used.