As an instructor I have looked at insurance options. Our organization has instructors that are independent and others that are dedicated to one shop. This does not present a problem for us as we are flexible in how we conduct our business of introducing others to the wonders of the aquatic world. I have retained independent insurance because I work for not only our organization but others. While more expensive, the other insurance is about ½ of what I now pay, it allows me to work with any shop or other instructor and not have to worry about crossing insurance lines. I would ensure that all those covered under the insurance you provide can cover those acting as part of your organization as well as independent training / working with others before I commit.
Just my .2cf worth