Subtle this book is not, neither visually nor in its overstated message. The familiar theme features a boy who overcomes his fears to attain manhood; in this case, a boy ventures into a swamp with his grandfather to feed an alligator that the old man saved as a hatchling from poachers. When he succeeds, the mantle of ""keeper of the swamp"" is passed to him from his ailing grandfather, who, the boy knows, ""wouldn't be with him much longer."" Garrett conjures up the eerie beauty of the swamp, with its ""tattered curtains"" of Spanish moss, but her descriptive powers are fettered by the contrived situation and general heavy-handedness (""Would he be big enough, would he be strong enough, would he be brave enough?""). The Day-Glo palette, however, almost irradiates the text; these colors approach the lurid, as does the combination of oil paints and digital images. The climactic scene, of the gator lunging for food that the boy holds out on a wooden rod, has the jerry-rigged look of a 1950s' monster movie. Ages 4-8. (Mar.) FYI: A Spanish-language edition is being issued simultaneously: El guardi n del pantano ($16.95, -13-2).