I have NBOA Boat Insurance for the boat and the jet skis.
https://www.nboat.com
The policy includes towing and you can use whoever you would like when you have a problem. You just submit the receipt after using them. I am not sure the towing companies will let you use other towing companies if they are busy.
At the time I started with NBOA they were the best and least expensive option for boat insurance. That was many years ago so I don't know if that is still true. It would probably be worth looking at what you are paying for insurance and towing combined and see if you can save money with an all in one policy.