Pound conch to 1/8-inch thickness, using a meat mallet or rolling pin. Cut conch into bite-size pieces; set aside.
Sauté onion in oil in a medium saucepan until tender; add flour, stirring well. Cook over medium heat, stirring frequently, until browned.
Add conch, water, potatoes, salt, and pepper; stir until well blended. Cook over medium heat, stirring frequently, until potatoes are tender. Serve immediately.