It's all about education and respect. I'm an instructor and UW photographer. Regardless of experience I always include in my briefing do not touch, tease or harass anything. I usually take this further with beginners telling them the ocean is not our home, we don't belong there and its only by technological advances we have the opportunity and privilege to witness its wonders. We should leave no trace of being there, ideally not moving as much as a grain of sand. We are mere visitors. If invited into someone's home as a guest we'd never dream of taking anything or moving their belongings, the ocean should be the same to us. Most are receptive to this, those who are not, I remind them politely that many things in the Ocean are venemous therefore for their own safety it is better not to touch. I find that usually does the trick.
With photographers, I give them tips on buoyancy and how best to get into position without disturbing anything. If it's evident their buoyancy isn't yet at a level they can manage that, then I'll never point out anything that's in an awkward place where they'll likely damage. If they do, then that's my fault for showing it in the first place.
So to answer the OP... Moving something may seem like no biggie, but these dive sites we all dive see thousand of divers there year in year out. If every other person moved stuff, then yes it is a biggie, it's a major biggie for the marine life there.
One of the sites I dive has had a Tiger tail seahorse on the same spot for 4 years. I've seen it photographed on a daily basis by many customers for all that time. I've seen clients with less than desirable buoyancy kick up the sand and corals surrounding it. It's heartbreaking. I won't even mention its presence to people unless their buoyancy is excellent. We have to take responsibility for our actions and their impact.
This year at the start of the season the seahorse isn't there anymore. Maybe it just died of natural causes but I'm secretly hoping it's moved elsewhere and nobody ever finds him again!
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