Having been in the pool/spa industry for over 30 years, I'll put my 2 cents in...
As for the liability waiver... I seriously doubt that AIRBNB's waiver will cover a host if the pool or spa is not VGB Act (2007) compliant and does not meet the requirements established for restricted access, ie fenced, gated. You may also be required to meet ADA requirements for pools and swim spas.
If you are not familiar with the VGB Act, here is a link to explain it.
https://en.wikipedia.org/wiki/Virginia_Graeme_Baker_Pool_and_Spa_Safety_Act
Pools and Spas built before 2007 ARE NOT COMPLIANT, unless you have specifically upgraded to meet the safety requirements established in that ruling. This involves replacing non-compliant equipment, drains, suction fittings with VGB approved ones that prevent entrapment.
The VGB Act was made enforceable in 2009. So if your spa or pool was installed prior to 2009, I strongly advise having a professional inspect it to insure it meets compliance.
Otherwise, you are inviting a potential risk, that most insurance companies will not support.
Understand that while homeowners with older spas or pools are not required to upgrade for personal use, Your spa or pool can no longer considered for personal use, when you open your home up to public use in a rental situation.
Keep in mind that in all public use spas or pools, the local, county or state health agencies are involved in insuring that health and safety regulations are being met. While you might not consider yourself liable for health and safety with regards to your pool or spa, it only takes 1 guest or tenant who contracts any number of diseases from using your facilities to make your litigation issues wipe out any waivers. You are responsible for the health and safety of your guests. Since pools and spas are potential breeding grounds for a multitide of pathogens, it is important to insure that the water is properly treated, by someone who understands the process.
Signage is a must! You have to provide safety rules, capacity notices and appropriate contact information near the pool or spa. If not than once again the waiver is useless. Yes, you might be to protect yourself against thinks like irresponsible behaviour, like drinking alcohol and then drowning in the spa, but these days, stupidity seems better protected than ever before.
Beyond that I guess the best response is cover your ass and hope you never have to litigate.
