In order to customize the booking calendar to only show specific products or open to a specific product, you'll need access the the Product ID to add to the booking calendar HTML code. The product ID is unique to the product.
To find the product ID, navigate to Setup - Products & Reservation Rules. In your list of products, select the product. On the Description page, you'll see the Product ID revealed in the URL (web address) seen in the browser.
In the example below the product ID for the Premium Pontoon Boat is 216849.