Miss Pickerell's the anchor-man again. . . aboard a floating classroom. Because Mr. Rugby's plankton are doing poorly, his ocean farm is failing. Miss Pickerell, who's sold on aquaculture and equipped with a diving suit, finds one remedy--stirring up the bottom--that turns out to be poison, then traces the poison--GREEN X--to waste disposal. Accompanying are her seven nieces and nephews and crusty Lieutenant Commander Cripes, U.S.N. Ret. and some accommodating oceanographers and a continuous survey of the field. This is less farcical than some of her jaunts but there is that built-in incongruity and lots of information.