Stockholders' and Members' Equity, Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020	Feb. 10, 2020
Numerator [Abstract]
Net income attributable to OneWater Inc	$ 20,475	$ 1,085	$ 28,263	$ 1,085
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)	4,251,000	8,589,000	4,327,000	8,589,000
Restricted Stock Units [Member]
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)	143,000	127,000	173,000	127,000
Common Class A [Member]
Earnings (loss) per unit attributable to common interest holders [Abstract]
Common stock, shares outstanding (in shares)	10,968,152		10,968,152		10,391,661	0
Denominator [Abstract]
Weighted-average number of unrestricted outstanding common shares used to calculate basic net income per share (in shares)	10,901,000	6,088,000	10,838,000	6,088,000
Effect of dilutive securities [Abstract]
Restricted stock units (in shares)	270,000	0	245,000	0
Diluted weighted-average shares of Class A common stock outstanding used to calculate diluted earnings per share (in shares)	11,171,000	6,088,000	11,083,000	6,088,000
Earnings per share of Class A common stock - basic (in dollars per share)	$ 1.88	$ 0.18	$ 2.61	$ 0.18
Earnings per share of Class A common stock - diluted (in dollars per share)	$ 1.83	$ 0.18	$ 2.55	$ 0.18
Common Class B [Member]
Earnings (loss) per unit attributable to common interest holders [Abstract]
Common stock, shares outstanding (in shares)	4,070,872		4,070,872		4,583,637	0
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)	4,108,000	8,462,000	4,154,000	8,462,000