There were several subfamilies of Mosasaur, we know of at least four through their fossil evidence. The largest growing to about 17 meters, or 57 feet, on average. Their massive jaw was hinged in a snake-like way, so that it could be opened wide to swallow large prey. It is believed that they used their tail as a propeller pushing them through the water.
Mosasaur is said to to be the T-Rex of the sea because it was the apex predator at the time. They were fairly widespread with Mosasaur fossils being found in New Zealand, USA, Denmark, and Canada (a huge find was made recently in Manitoba), as well as in other areas of the world that were once part of the giant sea that covered the planet 85 - 65 million years ago.
We can assume the Mosasaur ate what ever it wanted. A fossil found in South Dakota showed stomach contents of one Mosasaur containing fish bones, seabird bones, possible shark bones (sharks were alive at this time) and even the remains of an other Mosasaur. Shark teeth have been found in Mosasaur bones.
Although Mosasaur was a huge animal, it was not a dinosaur. Its legs attached to its body at the same angle as those of its nearest living relative today, the monitor lizard, to compare dinosaurs had legs that fit under (or infront) of its body, for example the tyrannosaur had legs that attached in a way we see in most mammals.
It has been suggested that the Mosasaur descended from the prehistoric snakes at the time and is a branch off of the snake classification.
T-Rex of the Seas Found in Manitoba
Most Prehistoric Animals were Not Dinosaurs
What Animals are Alive Now, and were Alive at the Time of the Dinosaurs