I hate regulation in any form, so my gut tells me no. BUT there are things to consider. First is the law. If they read and signed a waver are you, the proprietor of a kayak shop, owner of the rental property and holder of the rental contract really released from culpability if they are injured/killed? I'm no legal mind to be sure but I've heard that much of the time wavers don't mean much to the court system in instances like this. It probably varies from state to state.
If you truly are not liable for their actions after they sign, pay and leave your shop then it becomes a "moral" (I hate that word) matter much like deciding whether or not to approach PFDless Jim-Bob on the k-mart mattress ready to head down the play park at high water and tell him not to. If they read and signed and are presumably now aware of the risks and difficulty of the sport then maybe they should be responsible for their own actions. They are no longer ignorant.
In short, if YOU are not legally responsible for their life and you tell them they might die and they don't listen then I say take their cash and let natural selection go back to work. It's been too long on vacation.
I hope in the future Americans are thought of as a warlike, vicious people, because I bet a lot of high schools would pick "Americans" as their mascot. -Jack Handy