Alex,
If you were the trip organizer and director, making schedules and site choices, etc. it would be pretty hard for you to defend that you didn't give advice on the dives, despite having a certified and insured scuba instructor along.
The proper way to go would be to get divemaster certified and insured your self. That's what we had anyone in our dive club do, if they wanted to organize trips. You are covered for freediving as well, with divemaster or instructor insurance. I don't know, but would think freedive organizations like PFI and AIDA would have similar coverage today. I know Kirk is now offering instructor certification. See
http://www.performancefreediving.com/
My TDI/SDI insurance runs $560 yearly, and Divemaster was maybe $200 less.
Good luck.
Chad
