Great topic and a nice civil discussion. This is a topic I am always interested in debating. Being also an avid nature photographer above water it causes quite a stir when applied to those settings as well. The use of calls to attract deer or predators seems to be frowned on by a certain group of photographers as being a less pure form as setting in a blind waiting for hours for a subject to walk by. Also many nature photographers turn their noses up at photogs that go to game farms to get photos of nothing more than pets posed in natural surroundings and demand that photos should be captioned as such less they be confused with the "real thing".
As to the use of calls the objection always falls to the stress you are causing the animal as well as causing it to deviate from its normal behavior resulting in spending unnecessary energy it may not have to investigate the source or run off an intruder causing the subject , or its neglected young, do die a few days later.
I think one must always exercise caution to impact nature as little as possible but recognize no matter how careful you are you will always be impacting the environment in some negative way.
The photogs or even the environmental fanatics that believe that all these creatures are that easily stressed need to avoid all outdoor activity. While hiking to photograph scenics I have spooked deer and I am sure my scent has caused wild animals downwind to adjust their behavior. As someone said above just because you can point to another group that is doing more damage does not excuse your behavior. That poor feather duster that was feeding just retracted as you swam three feet over him might have to do that dozens of times daily in a heavily dived area causing him to stress out or miss crucial feedings making him more susceptible to disease or predators. If you mulitiply this over an hours bottom time think of the number of negative impacts a diver can have even when he does not get near the reef. ( not to mention all the microscopic plankton that his movement through the water is impacting, or does this life not really matter ?)
Based on this line of thinking diving should be illegal as we are gradually killing the reefs and their inhabitants. The bottom line is that as photographers we are all somewhat selfish and jealous at the same time. In the above example about the game farms the photographer that can afford to spend three months in the wild following the snow leopard and maybe get a one decent shot is upset that another photog with time/budget constraints get hundreds of beautiful posed shots of a snow lepoard in natural surroudings that only a small percentage of people might know the difference.
We are sort of a selfish bunch as we know all divers are creating some impact when just diving let alone photographing. So let me up the ante with Williams original question.... If you truly believe the talk of stressing out animals and killing coral, etc. why should you do it for nothing more than the pleasure you derive from it. Does the world really need another photograph of a pygmy seahorse to go with the ten of thousands that have been taken already ?