Combine 3/4 cup sugar, flour, and 1/4 teaspoon salt in top of a double boiler; stir in milk and egg yolks, mixing well. Cook over boiling water, stirring constantly, 20 minutes or until very thick and smooth. Remove from heat. Stir in 1/2 cup coconut and 2 teaspoons vanilla.
Place half of pineapple tidbits in pastry shell. Pour filling over pineapple.
Beat reserved egg whites (at room temperature) in top of a double boiler until foamy. Add 3/4 cup sugar, salt, and water. Place over boiling water. Beat at high speed of an electric mixer until soft peaks form. Remove from heat; add 1/2 teaspoon vanilla. Beat well. Spread meringue over filling, sealing to edge of pastry. Arrange remaining pineapple over meringue; sprinkle with remaining coconut. Serve immediately.