I am under the assumption DM's do not normally carry insurance thus the plantiff's thrust toward the boatowner and his watercraft insurance policy. It certainly seems as though most captains and/or crew would do their own count in simliar situations for obvious reasons.
That really depends on the DMs agency.
As a DiveCon (SSI) I can not purchase professional insurance because my status as a DM would be void if I were not leading a trip associated with the LDS I am associated with. I pay my insurance fees to my shop and am covered under their policy as an employee.
As a PADI DM, they are independent and each would/should have their own insurance. There may be casese where a PADI DM ONLY works with a LDS/Charter and is insured under the LDS/Charter policy.
Because the PADI DM is essentialy an "independent contractor" they should have their own liability policy. The policy would cover them no matter what chater or LDS they were working with at the time.
I can't comment on how any other agency works in regards to the DM