It's been a while since I played the game, but I think you must make them clear the beach of drift wood, then continue to put them in the water (adults only!) until they begin fishing. There is a good forum with hints on the website of the game developer. Hope this helps.
To get rid of sharks so your villagers can fish you need to create potions
NATURE POTIONS: 2 Black Orchids + 1 Pitcher plant = Gets rid of sharks
MAGIC POTIONS: 2 Roses + 1 part Berries = Gets rid of sharks
GameHouse Evangelist
Posted on Mon Feb 01 11:13:18 PST 2010