IF FURTHER TESTIMONY is needed to document the sea's vastness and diversity, these three books provide it, Each of them reports on a different family of sea creatures and does so in considerable - and often surprising - detail. Yet it is a measure of the sea's mystery that these scientific explorations conclude with fundamental questions still unanswered. While each of the books takes us to underwater horizons, seldom probed, each leaves us contemplating wonders still unknown beneath the surface of the oceans.

As a journalist and film-maker, Robin Brown, in his book The Lure of the Dolphin , knows how to order and compose a good story, and his hero, his star, is guaranteed box-office from the first chapter. In fact, the dolphin has always been a star among us - as Brown explains early on, quoting liberally from Aristotle and Pliny, both of whom, like thier fellow ancients, must have been keen observers of the sea's most abundant mammal.

For the Greeks of Aristotle's time, as for many other early civilizations, the dolphinwas more than a creature of interest - it was a god, on a par with the sun in terms of the stature of its deification. After reading Robin Brown's sparkling account of what has been learned about the dolphin during the past two decades by a number of scientists and observers the world around, one can certainly understand why.

With a larger and many times more complex brain than man, with a 15-million-year evolutionary jump on the "naked ape," the dolphin, as Brown establishes most delightfully, is far advanced in every aspect over us landbound beings. It is we who who should be learning from it rather than using it largely as a trained animal act to please crowds at dolphinariums, or slaughtering it heedlessly.

We are making, argues Brown, (and this is the book's central theme) a horrible mistake when we consider the dolphin just another animal. Instead, the author tells us (with much documented scientific supporting evidence) the dolphin is an alien on our own planet - a civilized, loving, communicative, frolicking, intelligent being who inhabits six-sevenths of our globe in a kind of utopian, non-violent, joyous society. Meanwhile the naked ape builds more destructive weapons and has begun destroying the dolphin, who, ironically, may be the only model we have for learning to live without conflict.

"To destroy them," writes Brown, "may well be the same as killing the only angel that knows the way to heaven, and an overt recognition of this accounts for the inexplicable attraction - the lure - that the dolphin has for man."

Among its many talents, the dolphin appears to be telepathic, and when I finished Brown's book I set about planning the mental message I would send the next time one surfaces at the bow of my boat - that's how convincing the author's incredible story becomes.

I will not try such messages with any of the sea turtles Jack Rudloe reports on in Time of the Turtle . Although he typically labels one which came his way a "big insensitive blockhead," this scientist clearly loves each of the seven species of sea-going turtles, and writes about them sympathetically. While his science sometimes interferes with his prose, the interest and compassion he feels for the creatures sustains the boook's momentum.

"All sea turtles cry copious tears," Rudloe tells us, and, after reading of their treatment at the hands of man, I can understand why. The loggerhead, the green turtle, the hawksbill, the leatherback, the terrapin, Kemp's and olive ridleys have - each in their time - been sorely abused and over-exploited. With their nesting places destroyed by bulldozers, their eggs stolen by poachers and their young suffocated by pollution, the sea turtles, killed by the thousands, may well be irretrievably sliding into the oblivion of extinction. For Rudloe the thought of such a loss is unbearable, and he persuades us that we must keep trying to manipulate Nature on the sea turtle's behalf. We must guard its nests, protect the few islands and beaches where its young are born, and outlaw its slaughter the world over.

Rudloe isn't quite certain why such efforts must be made; it is feeling rather than fact, he admits, which prompts him to such "unscientific" compassion. It is the sea turtle's eternal mystery - a mystery Rudloe explores and documents well and appealingly.

There is no such confusion for marine biologist Walter Starck who made the tapes Alan Anderson, Jr. uses as the basis for The Blue Reef . (Which arrangement, by the way, gives the book a lack of cohesion and adds some banal, talky prose.) For Starck, The reefs of Enewetak Atoll in the southern Pacific Ocean are alive with fish to be studied for study's sake. He chose the reef because buildings left on the atoll by the U.S. military - after the site was used for 49 nuclear weapons tests - still exist and provide lab space, and because the location is a favorite of the gray reef shark, whose attack behavior Starck wants to analyze.

His analysis involved a great deal of work with shark-killing "bank sticks," underwater explosives and chemical fish-killing poisons. Unlike Brown, who is convinced dolphins have much to tell us if we would only learn to listen, and Rudloe, who has come to feel a compassion for his "blockhead" sea turtles, Starck is quite convinced that Man rules the planet and has the right to "tinker" and "collect" and kill in the cause of empirical knowledge.

The sea is a large and mysterius place; in the cas of these three books, it turns out to be large enough to contain three quite different philosophies and value judgments. My agreement with Brown and Rudloe, however, is not my only reason for rating their efforts rather higher on the reader-reward scale. I do so because, by any objective standard, they belong there.

And of the two - dolphin or sea tutle - the dolphin is the charmer.