There's one thing that - AFAIK - hasn't been mentioned so far in this thread, and that's how many visitors a site or an area sees. A lot of my diving is in an area that at most may see a small handful of divers in a year. Some of the sites I dive are probably not dived by anyone else. In places like that, I have no problem with activities that affect the environment somewhat, like touching the sand, stabbing the occasional flounder, wolffish or monkfish, picking as many scallops as I want/need, catching crabs, or just picking up souvenirs. I probably have less impact on the environment than what the normal forces of nature have. If the winter storms can stir up the bottom at several tens of meters' depth, a single diver disturbing the bottom critters probably makes zero difference. And I have no issue leaving biodegradable trash like a little food waste or a few sheets of toilet paper as long as it's left out of sight, say like under a stone. It'll be gone and well into the biological cycle by the next time someone shows up. I am, however, careful to avoid disturbing protected or endangered species, or doing things that have a permanent or long-term impact on the environment.
OTOH, when I dive close to town or on a vacation abroad, on sites that I know - or suspect - sees a fair bunch of divers every year, I follow the "take only pictures, leave only bubbles" slogan. Because if I - and many others - behaved the same way on a frequently visited site as I do on a remote site, the impact on the environment would quickly become unacceptable.
"Sola dosis facit venenum"
