AS IF ON THEIR WAY TO NOAH'S ARK, A PAIR OF ENJOYABLE WILDLIFE documentaries march out Sunday evening (March 8). E.G. Marshall narrates Realm of the Serpent (ABC, 7 P.M. ET), part of the network's intermittent World of Discovery series. It's an often scholarly study of man's age-old fear of, and fascination with, snakes. Of course, this being TV, the emphasis is on big, nasty customers: pythons, anacondas, cobras and rattlers. Oh, my! Nature presents Dolphins: Close Encounters (PBS, 8 P.M. ET), which looks at our ongoing efforts to understand and communicate with these intelligent, ocean-going mammals. It includes a debate about whether all those programs in which people swim with or hand-feed dolphins are in fact damaging to these friendly creatures. In one experiment, dolphins avidly watch a TV showing footage of their trainer working at an adjoining pen in order to prove that they can differentiate images from reality and thus are capable of abstract thought. Seeing them glued to the set is rather droll. Wonder what they'd make of Regis and Kathie Lee?