Dissolve cornstarch in water in heavy saucepan; stir in sweetened condensed milk and egg yolks. Cook and stir until thickened and bubbly. Remove from heat; add butter and vanilla. Cool slightly. Fold in coconut.

Slice bananas; dip in lemon juice and drain. Arrange on bottom of pie crust. Pour filling over bananas.

Cover and chill pie for 4 hours or until set. Top with whipped cream. Garnish with toasted coconut and sliced bananas, if desired.