what did they eat?
Fish and squid.
Acamptonectes
Like other ichthyosaurs, it was a predator and probably fed on fish and squid.
what is acamptonectes? It is an extinct genus of ophthalmosaurid ichthyosaur.
wiki:19746948
Acamptonectes [SEP] Diet and feeding
As an ophthalmosaurine, "Acamptonectes" would likely have been an opportunistic generalist. Like other ichthyosaurs, it was a predator and probably fed on fish and squid. Their adaptations to speed suggests that these ichthyosaurs were pursuit predators. In 2012, the palaeontologist Maria Zammit suggested that the slender, shallow snout and tooth morphology of "Acamptonectes" indicated it had a different diet and lifestyle from other known Cretaceous ichthyosaurs. The slender tooth crowns with longitudinal ridges may have been used to impale rather than grasp prey, and its diet may have consisted of fleshy prey without a hard exterior. Icthyosaurs have been divided into "feeding guilds", and other examples include species that "pierced" small prey with needle-like teeth, and species that "crunched" hard-shelled prey with their more robust teeth.