I think many ichthyosaurs were more like modern beaked whales, which are mainly squid-eaters, although they also sometimes eat fish.
The argument with the fossilization is really good, I didn`t think about it. But there are several very good fossils from Holzmaden in Germany which shows not only all bones and the content of the stomach, but also the silhouette of the animals. If such a great fossilization happens, it would be strange if fish bones wouldn´t survive. Toady I was on the paleontological museum of Tübingen, and made some photos of the cephalopods there. I also looked at the ichthyosaur fossils, which had also all only relics of cephalopods in their stomaches.
Something about the Spinosaurus: In the stomach region of Baryonyx which was very similar to Spinosaurus, fossils of large fish were found, and the skull of this and other similar theropods like Suchomimus or Irritator were really so long and narrow, and also comparably small in relation to their bodies, that it is very hard to imagine that they hunted other dinosaurs. Furthermore their long claws on the fingers woulb be perfect to catch big fish, similar as grizzlies do it.
We still don´t know enough about the ancient ecosystems and the animals which belonged to it, to make really clear statements.