And on divers rude may be another diver reacting to what they my feel is inconsiderate behavior on your part that, in our own zen moment, we may not have been aware of.
Consider the example give by the op. She states she had been hovering for a few minutes watching a cool crab. Its possible in a hypothetical situation that the other diver had been wating patiently for their oppotunity to look, finally decided that it was her turn, and that carefully approaching below the other diver was a valid option.