Pat seafood dry; refrigerate while preparing base. Melt butter in a soup pot over medium; cook fennel and onion 8 minutes until soft.
Add potatoes, stock, salt, and pepper; gently simmer 15 to 18 minutes until potatoes are tender.
Add milk and cream. Whisk cornstarch with cold water; add and gently simmer 2 minutes until lightly thickened.
Add cod and shrimp; simmer very gently 3 minutes without vigorous stirring. Fold in scallops; cook 2 to 4 minutes more until cod and scallops reach 145°F and shrimp are opaque throughout.
Carefully divide among 6 bowls, distributing seafood and potatoes evenly. Serve immediately.