Stockholders' Equity - Summary of Calculation of Earnings per share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Numerator:
Net loss attributable to OneWater Inc	$ (114,582)	$ (5,705)	$ (38,592)
Class A Common Stock
Denominator:
Weighted-average number of unrestricted outstanding common shares used to calculate basic net (loss) income per share (in shares)	15,869	14,585	14,328
Effect of dilutive securities:
Diluted weighted-average shares of Class A common stock outstanding used to calculate diluted net loss per share (in shares)	15,869	14,585	14,328
Net loss per share of Class A common stock – basic (in dollars per share)	$ (7.22)	$ (0.39)	$ (2.69)
Net loss per share of Class A common stock – diluted (in dollars per share)	$ (7.22)	$ (0.39)	$ (2.69)
Class A Common Stock | Employee stock purchase plan
Effect of dilutive securities:
Restricted stock unit and employee stock purchase plan (in shares)	0	0	0
Class A Common Stock | Restricted stock units
Effect of dilutive securities:
Restricted stock unit and employee stock purchase plan (in shares)	0	0	0