Prep and cook time: about 30 minutes
1. In a 3- to 4-quart pan over high heat, bring about 1 quart water to a boil. Add potatoes. Cover and simmer until potatoes are tender when pierced, 15 to 20 minutes. Drain; keep warm.
2. Meanwhile, rinse fish, scallops, and shrimp; pat dry. Cut fish into 1/4-inch-thick slices about 2 inches long. Cut scallops crosswise into 1/4-inch-thick slices. Cut shrimp in half lengthwise. Arrange fish, scallops, and shrimp on a flat dish.
3. In a 3-quart metal chafing dish or pan over high heat, bring all the broth to a boil. If using a 2-quart fondue pan, fill it halfway with boiling broth (keep remaining broth hot; cover and set over lowest heat on a range).
4. Set container over an ignited alcohol or canned solid-fuel flame. Adjust for maximum heat. Place potatoes, seafood, rouille, and croutons alongside.
5. Spear potatoes or seafood, 1 piece at a time, on fondue forks or thin skewers (metal or wood) and immerse in broth. Cook potatoes until warm or hot, and seafood until barely opaque in thickest part, 30 seconds to 1 minute. Dip hot foods into rouille to taste, or spread rouille onto croutons to eat. If broth stops simmering, return to high heat on a range burner until boiling. Return to fondue burner. As broth in 2-quart pan evaporates, add remaining broth.
6. When potatoes and seafood are consumed, ladle cooking broth into bowls. Spread any remaining croutons with remaining rouille; eat while sipping broth.