Young potatoes -- often called new potatoes -- are a tender, tasty addition to this hearty seafood chowder.
Place clams and water in a medium saucepan over medium-high heat. Cover, and bring the water to a boil. Cook, stirring occasionally, until the clams have opened, 5 to 8 minutes. Remove clams, discard shells, and pass the broth through a cheesecloth-lined sieve, reserving the broth and clams separately. Set both aside.
Melt butter in a medium stockpot over medium heat. Add onion and celery, and cook, stirring frequently, until onion is translucent, about 4 minutes. Add flour, and cook, stirring, for 30 seconds. Add the reserved clam broth and the potatoes, bay leaf, and thyme. Cover, and bring to a boil. Reduce heat to medium-low, and simmer until the potatoes are tender, 10 to 12 minutes. Uncover, and add fish and shrimp. Cover, and simmer until cooked through, about 3 minutes. Add half-and-half and the reserved clams; season with salt and pepper. Heat through, but do not return to a boil.
The clams will not open at the same time. To prevent overcooking, check them frequently while they cook, and remove as they open.