First principle is: NOT ALL PLESIOSAUR NECKS ARE THE SAME.
Plesiosaurs had all sorts of different necks of different anatomy, different lengths and different flexibility. That is the outcome when you have any sort of adaptive radiation. It is absurd to make blanket statements that all Plesiosaurs had necks that were alike even if some necks might have been longer than others or less flexible than others (To select only two variables)
|Elasmosaurus scale, also shows neck in shallow vertical S-curve|
This ignores the possibility that the neck may be bent in the middle. However this is the practical reason why Elasmosaurs were stated to have necks which could not be held up out of the water. I have no problems with that.
We are not now and never have been talking in terms of the Elasmosaurs with extremely long necks!
All we have ever done (from Gosse on) is to suggest that we are dealing with a very large kind of Plesiosaur with a moderately long neck (less than twenty feet ): "Essentially a Plesiosaurus with the colossal magnitude of a Pliosaur" as Philip Gosse said in 1860
Romance of Natural History on Googlebooks