When I freedive I can hear scubadivers by their loud sound from far away; even when I cannot see them underwater nor their boats on the surface. Then, I change location because I know I won't be able to catch any fish where they are.
Even the slightest noise can scare fish, not to say marine mammals like whales. Of course I'm talking of those in open waters, not in marine reserves where they are used to humans.
I've been swimming with dolphins in the Mediterranean but they were very shy. Friends of mine have been freediving with common rorquals (2nd largest whale in the world, 75Ton) also in the Med. Their pictures which I'm no sure I'm allowed to share are just breathtaking. We, the human beings are so small compared to whales!
Also, a friend of mine has a rebreather. It's quite an expensive equipment and needs a special training in order to be used. They are quite complex too compared to regular scuba. Besides making no bubbles, rebreathers allow longer dive times. It's the best way to approach marine life without scaring all animals around besides freediving.