First, I'll show you a diagram of a generic plesiosaur. You can see the hydrodynamic body, composed by a small head, long neck, and compact torso, and equipped with four fins.
See also how I divided the spine in three curves with different directions. While these curvatures certainly changed in different positions of the animal, this is the 'rest' pose.
Remember that there was a great diversity of plesiosaurs (for more info, visit www.plesiosauria.com), so it's okay if you modify a bit the proportions.