Since in the Derby Marking is "ALL" important.
First series Land, all dogs that did not Handle or Pickup or have any other cause for elimination fault, would get to run the second series which would be water marks by Mandate .
By doing so all dogs would have demonstrated their Marking ability or lack thereof on both land and water.
At that point evaluate all of the dog work and drop any dog whose remaining work, if perfect, when compared to the rest of the field with similar work in the last two series, would not get a placement.
Do the third series , probably land, similarly evaluate then go on to a second water series.