First came Jules Verne at 20,000 leagues. Then there was the Little Mermaid. Today, thanks to Jim Toomey, it's Sherman's Lagoon, a satirical, sea-floor look at popular culture as showcased through the lives of a lovable shark and his waterborne cronies. Sherman's the somewhat dim-witted but happy-go-lucky shark who takes us for a wonderful ride beneath the waves. This Jaws-less jokester teams up with a veritable school of bottom-dwelling denizens to deliver one of the funniest creations on land or sea. With an imaginative storyline and creatively rendered characters, Sherman's Lagoon has captured a considerable following.   Sherman--who never allows thinking to interfere with life's simple pleasures, especially eating--is joined by Fillmore, his trusty turtle sidekick; Megan, his significant shark other; Hawthorne, the irascible hermit crab; and a host of other Neptunian neighbors occupying the lagoon of an imaginary South Pacific island called Kapupu.   In Sherman's Lagoon: Ate That, What's Next?, this cast of coral reef critters never fails to amuse. Consider Sherman opening a wrapped holiday gift box only to find a putrid dead fish--and loving it! Or the day he and Fillmore contemplate plunging to the deepest depths of the ocean, in order to recover Fillmore's fumbled Ninja Turtle Decoder Ring. The fun and laughs never end!