Tyler may be right waiver v disclaimer, but I am absolutely certain, UK law Unfair Terms in Consumer Contracts: that the waiver or disclaimer is not worth the paper it is written on IF the instructor is negligent. You cannot introduce a term in a contract (ie the student agrees not to sue the instructor if the student gets hurt) if that term attempts to limit the instructors liability if the instructor fails to safely instruct and by any NEGLIGENT failure causes harm. You cannot contract your way out of negligence.
The question is whether the instructor A) was negligent and B) more importantly, whether the student can prove it.
This is why the student must be insured by the course organiser. It is not good enough that the instructor insures himself only and then attempts to unlawfully waive the rights of the student by shoving a carefully worded disclaimer under their nose at the commencement of the course.
Effectively, you can see the instructor causing the accident and saying to the student afterwards "tough luck you signed the waiver and cannot sue me for your devasting injuries". The waiver will not stand up in Court because it is an illegal document.
What might constitute negligence in a freediving accident. How about no oxygen on the boat? The question relating to minor accidents is interesting though. When I was in xxxxxxx on a freediving course, the session had just ended and we had motored back to the "marina" wall. The instructor placed the bow of the boat against the wall at a 90 degree angle, so we could all step off the bow onto the wall. I carried a couple handfulls of kit and stepped off the bow. Unfortunately at the same time, the instructor turned of the motor. This caused me to kick the boat backwards with my step and rather than propel me forwards onto the wall, I simply stepped straight into the water. Kit floated away and we all had a great laugh. I didnt notice the blood coming from the tip of my nose immediatly. But a chunk of skin had been grazed down the wall, it was that close. However, if I had managed just 1 inch further, the wall would have smashed the complete underside of my nose and the bones in my nose could have penetrated my brain. If I had managed 4 inches further, my chin would have taken the impact and maybe broken my neck. Of course 6 inches further and my foot may have made first contact and all would have been well.
The issue is, how on earth do you sue your best friends who may already feel stupid for contributory actions causing your accident. Forget the disclaimer for a while. This is why insurance must be watertight. My message is again to instructors to insure themselves and their students. Then in the case of an accident, the instructor can breath a sigh of relief knowing that despite the cause of the accident, despite his or the students stupidity, the student will have some cover, without having to prove who was negligent and who was liable. The insurance simply pays out.