As I'm reading the 2009 standards, this is my take on it.
Dive guides can only give underwater tours to certified divers, teach snorkeling only after they have passed the Snorkel Instructor module, and issue Snorkel Diver certifications (page 37 of the 2009 standards). They may NOT conduct skills update, assist SSI Instructors with academic, pool or open water training. May NOT take students in training on underwater tours, teach or issue OWD certifications, or act as a certified assistant.
The SSI Divemaster is NOT comparable to any other agencies Divemaster and really they should look into changing the name to avoid confusion. A quote from the 2009 standards, "The Divemaster rating is a recognition level. Therefore, Divemasters have the same rights and limitations as certified Dive Guides."
As far as insurance goes, until SSI comes out with something written (insert opinion) I am assuming that anyone qualified to lead groups on dives (i.e. be responsible for other divers in a professional capacity), teach and issue certifications on ANY type of training (snorkeling) would require insurance.
EDIT: Just found the part in the standards that mentions where Snorkeling Instructors must have insurance, but it doesn't mention anywhere else that if leading a group (underwater tours) you have to have it.