Books about the sea are like waves along a beach; they keep rolling in. As every good surfer knows, not every wave is worth riding, but in the past few months at least four books about the sea and its shores have appeared that will reward a reader's plunge.

In the most basic of them, The Ever-Changing Sea (Knopf, New York, $7.95), Authors David B. Ericson and Goesta Wollin deal for the most part with the latest oceanographic findings of the Lamont Geological Observatory of Columbia University of which they are both staff members.

Their emphasis is on the physical aspects of the sea, with chapters on circulation in the ocean, changing sea levels, wave formation and the face of the earth beneath all the water. The book is likely to be of more interest to the student than, say, to a fisherman (though there is a chapter on life in the depths), but it is well written and highly informative. One chapter concerns the well-publicized Mohole Project, in which scientists hoped to drill a hole 15,000 to 30,000 feet deep into the ocean floor until Congress scotched the project.

In The Frail Ocean (Coward-McCann, New York, $5.95), Wesley Marx declaims on the spoliation of the sea and its coastal regions with all the fervor one might expect of a man bearing the names of two ideologues. He tells of a mysterious paralysis that afflicted a Japanese who ate seafood from Minamata Bay and of how subsequent investigation showed the ailment was caused by the toxic mercury effluent discharged into the water by a chemical factory. He discusses the dying kelp beds of the California coast and blighting of many estuaries. Overall, his book is something of a mishmash, but it does contain much of interest and may be of service to those seeking to stop further corruption of the waters around us.

Along this line, John Clark, president of the American Littoral Society, has compiled a paper-bound handbook called Fish & Man that outlines the problems of the Atlantic Coast. Clark warns, as have many before him, that the valuable fishery resources of the coast are in danger of extreme depletion from poorly planned community and industrial "development," and he gives a state-by-state rundown, from Maine to Florida, on the problems and the adequacy or inadequacy of state laws to meet them. Clark is a biologist and, in fact, assistant director of the U.S. Bureau of Sports Fisheries and Wildlife Gamefish Laboratory at Sandy Hook, and he knows the problems first hand. Fish & Man costs $1, and copies of it may be procured by mail from the American Littoral Society, Sandy Hook, Highlands, N.J. 07732.

Finally, there is Marjory Bartlett Sanger's World of he Great White Heron (Devin-Adair, New York, $10), which is not so much about the marvelous bird Audubon first made known to science in 1832, as about the natural and human history of the waters in which it lives and the islands that dot them: the Florida Keys.

"Like pearls on a string," Mrs. Sanger writes, "the islands of the Florida Keys curve from the Everglades to Key West. On and around them, above and beside them, whole colonies of animal and plant life inhabit the reefs and shallows, the beaches and hammocks, the sea and the sky. Something is lost if one surveys the landscape with its shimmering, multicolored and changeful beauty and has not some conception of its teeming, and usually hidden, wildlife communities."

Mrs. Sanger writes of the life of the Keys in all its many forms. She writes of the first Spanish explorers, shipwrecks (residents of Key West reaped as much as $1.5 million one year in salvage), murderous plume hunters, Conchs and Cubans, the storms that ravaged the chain, and, of course, John James Audubon.

Audubon was already famous when he arrived at the Keys in the revenue cutter the smugglers called Lady of the Green Mantle. "It is no secret," says Mrs. Sanger, "that Audubon had been disappointed in northern Florida....

"But from aboard the Lady he gazed with 'delightful feelings' on the jade water of Florida Bay with its bands of cobalt and citron and apple-green, its milky marl and sculptured reefs and coral sand, its 'flocks of birds that covered the shelly beaches' and the countless islands the Spaniards had called cayos, 'little isles.' "

There is an enthralling chapter on Henry Flagler's effort to build the railroad to Key West. In 1906 more than a hundred railroad laborers were killed by a hurricane; three years later 40 more died in another storm, but Flagler finally completed the line in 1912 and rode the first train in celebration. "He had been eighty-two just twenty days before. He was frail, with eyesight that had failed badly. But no matter how it felt about the railroad, the city gave him a gala welcome. Conchs and Cubans crowded into the station on Trumbo Island. Many of them had never seen a train before. A thousand school children threw rose petals at an already legendary figure who wept because he could not see them. It is pathetically easy to identify Flagler in the photographs of the festivities. He is the one holding the big white handkerchief to his face." Artist John Henry Dick has contributed 43 superb line drawings to help make Mrs. Sanger's book one of the most rewarding of the season.