We own a boat rental and I have a question on how to categorize our rental equipment such as tubes, mats, skis, etc.

QB does not do rentals.

Those items are all inventory items

but you need to use a service item for the rental, other wise the cost of the items will post to COGS every time you rent it.  Of course you could use inventory to rent it, then use a credit memo to record the item coming back to stock.

Its your choice

Whether you rent out, buy for resale,, or use internally makes no difference. It is all fixed assets of an inventory nature. Crrtainly on a periodic basis you count the numbers. You may even have assigned numbers to each rental piece. Each category of product rented has an expected lifespan, determined by the IRS, over which time you expense your purchase through depreciation. Regular repeatable turnover may qualify for safe harbor de minimis up front deduction.