Bring water and rice to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until rice is tender and liquid has been absorbed, 20 to 25 minutes.
Place rice and 1/2 can clam juice in a blender; puree until smooth.
Melt 2 tablespoons butter in a large stock pot over medium heat. Add onion, ham, and garlic; cook and stir until garlic starts to pop, about 10 minutes. Mix in potatoes, salt, and pepper. Add the remaining 2 1/2 cans clam juice. Cover the chowder base and cook until potatoes are fork-tender, about 15 minutes.
Dice shrimp into small pieces. Add shrimp and scallops to the chowder base. Cook until shrimp turns pink and scallops are opaque, about 5 minutes. Stir in the rice puree, the remaining 2 tablespoons butter, corn, garlic powder, and onion powder. Reduce heat to low. Simmer, covered, until flavors combine, about 5 minutes.