Hi Melissa,
This is how I've been handling it for years.
Airbnb requires Guests to agree to all House Rules, so I have a House Rule that states: "11) Guests agree to read, understand and adhere to the Liability Waiver at the end of my listing Description."
Then I have a long liability waiver at the end of the listing description. It's too long to fit in the Description box, so I had to drop it into "The Space" box, but it shows up in the Description for guests who are looking.
I've never had a problem, and if I do have one in the future, I can say that guests agreed to the House Rules before booking, as required by Airbnb. So, they actually don't sign anything, I'm relying on their agreement with Airbnb to adhere to my House Rules.
Here it is:
Liability Waiver:
You understand that swimming, hiking and the use of kayaks is at your own risk and that special risks may be involved with swimming, hiking and using the kayaks. You accept full responsibility for any bodily injury arising from swimming, hiking and use of the kayaks and hold owner harmless. You understand there are potential dangers that swimming, hiking and the use of kayaks may present to children who are not carefully supervised, as well as the danger to any person swimming, hiking or using the kayaks if a person has health risks, or if a person swims, hikes or uses the kayaks while intoxicated or using any kind of drugs or medication, or while pregnant. You agree to be fully and solely responsible for any accidents you may incur. You understand the risks discussed above and agree that you will assume all responsibility for yourself and for the consequences of those in your party. You agree to waive any claim whatsoever against owner for accidents or claims arising from swimming, hiking or using the kayaks. All Airbnb Guests in your party acknowledge and agree that they have read and understand this waiver and they acknowledge that booking constitutes a binding and enforceable contract between Airbnb Guests and Airbnb Host.