Helen Hayes just doesn't cut it.
Rather than going the whole hog and playing Miss Marple as an American, she plays her as an Englishwoman. This is a mistake,since her accent veers from deep south of USA to England via Ireland. In short, her accent is all over the place.
Her lines are also peppered with Americanisms which no British person would ever say. Two examples:
1) She refers to "tourist class" when British people call it "economy class".
2) She says she's going to "mail" some postcards, when a genuinely British person say "post", not "mail".
Two minor examples, I know, but they add to a general feeling that this Miss Marple is as British as a fudge brownie. All the references to her hometown of St. Mary Mead in England can't change that.
Another point is that Santa Barbara is not in the slightest bit convincing as a stand-in for the Caribbean. The one shot of a caribbean town, Havana perhaps, is obviously grainy archive footage.
Steer clear of this poorly made rubbish, and watch the BBC productions starring Joan Hickson instead.