Carol, it sounds like your tub has dropped over time, or the original installation was done poorly; there should never have been that much space left open.
Short of redoing the entire lower section I can only think of one method that I might use. This would be to have a 3" or 4" "backsplash" of Corian-type material made up that would adhere to the face of the lowest tile and the edge of the tub. This would be siliconed in place, and as long as you had a good tight fit and seal at the top edge it would resolve the problem.
Good luck, and post back if you have any further question.