Strain clam juice through a fine-meshed sieve lined with a double layer of cheesecloth. Set aside. Separate the hard part of the clams from the soft. Chop hard portion finely; set soft portion aside. Place salt pork in a large soup kettle. Cook over medium heat until pork has rendered some of its fat, then add onions and cook, stirring often, until onions are limp. Add chopped hard clams, potatoes, and water; simmer gently until potatoes are tender. Season lightly with salt and pepper. Scald milk in a separate pan. Cream butter and flour to form a paste; add to hot milk. Add clam juice and cook, stirring, over low heat, until smooth. (Do not allow to boil.) Add crackers and pour into clam and potato mixture. Add soft clams; blend. Correct seasoning with additional salt and pepper as needed. Serve very hot.