Yes, I watched that one back around the time I read the book! Perhaps Helena Bonham Carter's verve and spunk blinded me to Helen's weaknesses. (Also, IIRC in the 1992 production Helen and Leonard Bast did it in a boat beneath the rippling leaves of a weeping willow - my memory may be embroidering this somewhat - but nonetheless the inclusion of the boat made it far more understandable that they both lost their heads.)