Oceans - 6 free documentaries about dolphins & porpoises.
Start with Ocean Stories 3 - Dolphins and Whales (49 min, Free Documentary), then Samu the Baby Dolphin (24 min, BBC Earth). All 6 documentaries here are free to watch from their makers' official uploads. To read deeper, the top-rated book on dolphins & porpoises is Voices in the Ocean by Susan Casey.