The most likely cause of death is drugs mixed with alcohol. Kid(s) drag Dad in. Blood being thicker than water (and Dad's legal training tells him that no body means most likely means a circumstantial case), Dad became an "accessory after the fact" (though that may not be the Dutch charge) and helped the kid(s) dispose of the body. Sharks are a great help...but some body parts will eventually appear (I hope). Also, hair (except for on my noggin) is ubiqitous. If she was in a car trunk or boat, Natalee's hair is in their somewhere.
But, truth being so much stranger than fiction. I am probably off by a mile.
Death from a mixture of drugs and alcohol would be an accident--manslaughter charge...not PREMEDITATED MURDER.
But I am often wrong in my predictions of how these cases will go.