It would be nice to integrate beach gear rentals in the guest checkout process. I'm not aware of a way to do this today other than including a message telling them to rent beach gear elsewhere.
For my location on Hatteras Island, I spoke with Ocean Atlantic Rentals and they seem interested in a partnership for beach gear rentals when guests make their booking. They provide equipment for 100+ miles along the NC Outer Banks including Hatteras Island.
Is this something OwnerRez would be interested in?
The OwnerRez surcharge system, found in Settings > Surcharges, gives you a way to offer these services to your guests to select on checkout. The option add-on will be added to the list of charges, and will be paid for according to the usual payment schedule along with everything else. Check out the support doc below to learn more:
You can also take a look at The Host Co which is an upsell platform that allows you to create online stores where guests can purchase add-ons, services, items, etc.