This is the most important rule of freediving but is often misunderstood. A proper dive buddy is one that is a trained free-diver who knows rescue techniques. This means that a group of snorkelers you are swimming with are not buddies. Neither is a group of scuba divers below you. If you blackout at depth, most snorkelers will not be able to reach you, and scuba divers cannot bring you up without risking their own safety.
Avoid accidentally kicking other divers, damaging coral, and kicking up sand when other divers are around.
And most importantly: mind your farts