A year or so ago, The science channel (if I recall correctly) did a special on the design and construction of this water slide. They had lots of design challenges and initial testing proved that the “rafts” tended to go airborne at the top of the initial drop and the top of the second drop. They modified the angles of approach and put nets over the areas. Sounds like maybe something like an airborne raft may have recurred. Perhaps it was too light weight with a 10 year old in it. So sad.
I hadn’t really considered aerodynamics. While a heavier raft would tend to be faster (I figure), a lighter raft would be more prone, once it lifted off the surface of the slide, to being lifted by the force of the air underneath it.