From the legal eagles:
The National Park Service does not allow businesses that operate within the Park to require guests to sign a liability waiver form for accidents or injury occurring on NPS property. However, requiring that guests sign a form or statement acknowledging risk and/or indicating that certain prerequisite skills may be needed to participate in the activity is allowed. Requiring a liability waiver form or statement for accidents or injuries outside NPS property is allowed. Therefore, every reservation requires acceptance of such a form.
That document, which is presented in full during the booking process and requires acceptance by checking the agreement box, states more fully what is summarized here.
In short, the customer acknowledges all risks for injury and death of all members of their group, and all risks of property loss and/or damage while within NPS boundaries, and acceptance of all those risks outside the NPS boundaries.
In addition, the customer agrees that the processing of the rental transaction with a credit card or PayPal account is explicit agreement to the terms.
Such agreements and waivers are part of modern life and should not come as a surprise to anyone. The terms are non-negotiable, but All About Water will be happy to answer questions about the document.