Who is the Watson for Miss Marple?

She doesn't have one. Agatha Christie used the Holmes-Watson paradigm when she wrote her first book, The Mysterious Affair at Styles, by having Hercule Poirot have a sidekick in the person of Arthur Hastings. By 1930, ten years later, she was an assured enough writer that she did not feel the need to copy Conan Doyle's example.

She does have assistance in a number of cases, especially in "What Mrs. McGillicuddy Saw" also known as 4:50 from Paddington, where she hiredLucy Eyelesbarrow to be her eyes and ears on the scene.