Didn't I read that the divemaster was supervising 20 divers in the water at the same time? That seems a bit much for any one divemaster.
SeaRat
I don't understand why you would think a divemaster supervising 20 divers who present themselves with proper credentials is too much. The divemaster is not responsible for every aspect of an individual diver's safety unless they are specifically contracted to do so or acting as support for non-certified divers in a training scenario.....the latter was not the case here.
As a certified diver, freshly minted or otherwise, one should be competent enough to execute a dive safely in an environment and conditions similar to their training...beyond that, assuming the conditions were not similar to the OP's training, it is expected as well, that given the high risk nature of diving as an activity, that each certified diver be able to make a basic assessment of their ability and comfort relative to any dive they may conduct. Certified divers should be expected to respond based on the protocols learned during their training if/when they find themselves beyond their comfort zone, personal limit, etc. and exercise the option of terminating the dive early if it is warranted or desired.
The OP had 20 dives under their belt, that is 15 past what is necessary for certification, and while that is nowhere near enough to be considered an expert in any sense, it is enough to expect a diver to have an awareness of basic factors during the dive along with how they should respond when those basic factors are not unfolding per normal during the dive.
Perhaps there is information that might be missing such as the information included in the dive brief that divers often ignore and find themselves participating on dives beyond their ability and comfort level. What is clear, and I am not trying to assign blame here, the OP allowed themself to be stretched past their comfort zone and did not follow the protocol of their open water training. It is too easy to blame the divemaster, but I think that the majority of the onus is on the individual diver to recognize/understand/know/ their limits and remain within them.
SCUBA diving is an inherently high risk activity as it is conducted in a medium we cannot naturally survive without specialized equipment and training. Becoming certified to dive (as an adult), endows the individual with basic knowledge to take responsibility for the choice to enter the water and participate. Accepting the risk(s) that come with entering the water is ultimately the responsibility of the individual diver.
-Z