Growing up Ichthyosaurus was a catch all, in the same way that mosasaur and plesiosaur have been used in the past, for any fish-like reptile between the size of dolphins and orcas. The older, more fanciful Ichthyosaurus of Harder had more fish-like structuring to its tail and other body elements. All said and done, there really was not much about the body of Ichthyosaurs that Harder, and other early illustrators and paleontologists, got wrong. Ichthyosaurus was a highly stylized and fashioned reptile. It was also extremely well adapted and, over millions of years of tweaking and successive adaptations of the body plan of the animal, became, roughly, the equivalent of modern dolphins and orcas in terms of agility and predatory nature. Despite a diet that consisted, evidence supports, mainly squid, this was a fast and acrobatic highly adapted reptile that was entirely marine in habitat and behavior.
That body plan of speed and agility remained nearly unchanged once adapting to what we might consider its pinnacle or most well adapted form. Using evidence, and I sound a bit repetitious here, we know that the body of Ichthyosaurs, in their height of marine dominance, was that akin to a dolphin's and we have evidence, to be shown shortly, that they gave live birth in a manner reminiscent of modern marine mammals. As such, we can make some general assumptions, essentially educated guesses, pertaining to the behavioral structure of Ichthyosaur life history. Understanding their diet, life cycles, and body plans, it is safe to say that Ichthyosaurs would have been comfortable in groups and that it was probably normal for the animals to exist in some sort of social structure akin to, and we may even call it, a pod. Such a grouping of animals would have allowed for cooperative rearing of young, hunting, and defensive strategies which would keep the group alive.
One such strategy, of course, would have been safety in numbers. Against smaller predators, or perhaps similar sized predators which attacked in groups in a similar manner to the Ichthyosaurs themselves, a group showing aggressively and posturing, would allow for a coordinated defense that would either end up in a physical struggle or one side or another backing down escaping to the best of their ability from what we would consider the victorious side. Against a larger predator, such as this Thalattoarchon saurophagis, which is actually a more basal Ichthyosaur from as early as the Triassic; it kind of makes the image of it hunting down its more "advanced" cousin ichthyosaurs fairly humorous. However, considering that Ichthyosaurus was a numerous and highly successful genus by the time of the mid Jurassic it would have been a hot meal item for any larger predator and, the safety in numbers strategy partly states that one death can save many lives.