Stockholders' and Members' Equity, Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020	Feb. 10, 2020
Numerator [Abstract]
Net income attributable to OneWater Inc	$ 34,503	$ 14,367	$ 62,765	$ 15,452
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)	4,264,000	8,754,000	4,345,000	8,697,000
Restricted Stock Units [Member]
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)	201,000	292,000	221,000	235,000
Common Class A [Member]
Earnings (loss) per unit attributable to common interest holders [Abstract]
Common stock, shares outstanding (in shares)	11,661,575		11,661,575		10,391,661	0
Denominator [Abstract]
Weighted-average number of unrestricted outstanding common shares used to calculate basic net income per share (in shares)	10,976,000	6,088,000	10,884,000	6,088,000
Effect of dilutive securities [Abstract]
Restricted stock units (in shares)	365,000	9,000	259,000	5,000
Diluted weighted-average shares of Class A common stock outstanding used to calculate diluted earnings per share (in shares)	11,341,000	6,097,000	11,143,000	6,093,000
Earnings per share of Class A common stock - basic (in dollars per share)	$ 3.14	$ 2.36	$ 5.77	$ 2.54
Earnings per share of Class A common stock - diluted (in dollars per share)	$ 3.04	$ 2.36	$ 5.63	$ 2.54
Common Class B [Member]
Earnings (loss) per unit attributable to common interest holders [Abstract]
Common stock, shares outstanding (in shares)	3,377,449		3,377,449		4,583,637	0
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)	4,063,000	8,462,000	4,124,000	8,462,000