NATIVE TONGUE By Carl Hiaasen (Pan, $79) IT'S hard being funny. It's harder still being funny all the time. Carl Hiaasen tries, gets halfway there, realises he has used up all his jokes, and promptly falls over his own feet in a desperate hunt for another punch-line. Native Tongue is a comic novel, but ultimately Mr Hiaasen is hoist by his own petard. He sets the parameters - that every page should be funny - then finds himself hopelessly confined by them. His fall guys are two hapless crooks called Danny Pogue and Bud Schwartz. The heroes are a disillusioned journalist, a violent granny and a politician with scruples, who has dropped out and lives in the wilds of Florida dispensing his own brand of justice. The villain, of course, is a fat, blubbery businessmen; a poor man's Robert Maxwell. He is easy to hate because he was produced not by the imagination but a formula which goes: fat businessman equals horrible person. At times, this book seems as naive as pantomime. And therein lies the problem. Stereotypes are rarely funny and custard pies in the face are a thing of the past. Novelists have to do more than write the words ''wet willy'' to make people laugh (the wet willy water slide being a star attraction at the crooked businessman's theme park). The plot is an admirable attempt to deliver a figurative kick in the crotch to two of modern America's greatest evils: theme parks and the game of golf. It never quite succeeds, but along the way it does show touches of creative power. Former journalist Joe Winder is now a public relations executive, working for the Amazing Kingdom of Thrills, a theme park owned by millionaire Francis X. Kingsbury, the fat villain. Molly McNamara is a gun-toting granny and an animal rights campaigner. She suspects nasty goings-on at the Kingdom. Other characters include a sexually deviant dolphin, a man-eating pet whale and Robbie Raccoon, with whom Winder falls in love. Whacky indeed. But whacky is not enough. Carl Hiaasen can be very funny. But comic writing still needs characters, Tom Sharpe's Blott, or John Mortimer's Rumpole, for example. Mr Hiaasen tries, but armed only with a cast of cartoon outlines, doesn't come close.