Significant Accounting Policies - Summary of Revenue Disaggregated Into Categories Depict the Nature, Amount, Timing, and Uncertainty of Revenue and Cash Flows Affected by Economic Factor (Detail) - Product Concentration Risk [Member] - Sales [Member]	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	100.00%	100.00%	100.00%
Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	100.00%	100.00%	100.00%
Product Manufacturing [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	100.00%	100.00%	100.00%
New Boat Sales [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	68.70%	73.20%	70.50%
New Boat Sales [Member] | Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	67.30%	71.90%	70.30%
New Boat Sales [Member] | Product Manufacturing [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	95.80%	95.00%	98.80%
Used Boat Sales [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	7.90%	7.30%	10.90%
Used Boat Sales [Member] | Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	8.30%	7.70%	11.00%
Used Boat Sales [Member] | Product Manufacturing [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	2.80%	3.80%
Maintenance and Repair Services [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	4.40%	4.10%	4.10%
Maintenance and Repair Services [Member] | Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	4.60%	4.70%	4.10%
Storage and Charter Rentals [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	6.70%	3.30%	3.00%
Storage and Charter Rentals [Member] | Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	7.10%	3.00%	3.00%
Finance and Insurance Products [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	2.80%	3.00%	2.70%
Finance and Insurance Products [Member] | Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	2.90%	3.10%	2.70%
Parts and Accessories [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	4.70%	3.30%	3.20%
Parts and Accessories [Member] | Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	4.90%	3.50%	3.20%
Parts and Accessories [Member] | Product Manufacturing [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	0.80%	0.70%	1.20%
Brokerage Sales [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	4.80%	5.80%	5.60%
Brokerage Sales [Member] | Retail Operations [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	4.90%	6.10%	5.70%
Brokerage Sales [Member] | Product Manufacturing [Member]
Product Information [Line Items]
Sales Revenue Goods And Services Net Percentage	0.60%	0.50%