It always bothers me when instructors make you sign a waiver before a course. Not just freediving, but virtually any course. I believe you instructors are the experts and if you cause through negligence: ie parachute instructor not clipping in the static line, driving instructor forgetting to remind the student to put on the safety belt, flying instructor not telling you to approach the aeroplane from the rear so you dont walk into a rotating prop, freedive instructor........(you fill in the blanks), and your student hurts themselves because of your lack, then you are liable, even if your student signed a document saying they release you from liability.
Then we have to ask ourselves, is it fair for the student who is now injured, to have to prove you were negligent, before they can be compensated for your mistakes or forgetfulness or down right dangerous methods.
I am not certain that the signed waiver will protect you and that is why you must be insured. Then again, is there anything in your insurance policy that restricts a pay out, particularly if you are shown that the accident was due to your negligence. However, even if the accident was the instructors fault, it is still a question of whether that fault was an accident or a negligent act. Its a real minefield and I have signed so many waivers. The problem is that the waivers are generally handed out during the first morning of a course, its difficult at that point to decide not to sign and fly home instead. And if they did decide not to stay, would you reimburse their course fee?