I belong to BoatUS which includes towing, BUT I could not get anyone to tow us. My boy called from a land line after I texted him on our InReach and was told they didn’t cover Powell? We towed the down boat a couple years ago to Wahweep from the SJ. When I got home I called to cancel my subscription and was told they do cover Lake Powell but I have yet to find anyone who represents them. My best advice is count on yourself or have an INReach or some other device that will save your bacon. We have had our share of issues over 50 years but so far no one has ever baled me out, we got ourselves back and out of trouble, be prepared!
Edit: we were told they would come from Wahweep a couple years ago when we had trouble, in twelve hours and it was around $1500 to tow from the SJ back to Wahweep. They did not honor BoatUS. My family would get there quicker and much less money (nothing) coming from 5 hours away plus boat time. Be prepared to help yourself if you go far away from the marinas, that’s where the croc’s are!