they arent dinosaurs, and they are various unrelated groups of animals.
sorry to burst a simplicity-bubble, but theyre very complex little groups of animals.
one is the Ichthyosaurs, these are the dolphin-like reptiles.
then theres the Plesiosaurids and the Pliosaurs, theyre both related to each others, and are long necked and short necked marine reptiles. the long necked ones are known for that "nessie" shape.
then there are Mosasaurs, these are related to snakes and lizards.
unfortunately, these 3-most-famous marine reptile groups arent related to each others, and youll most likely not find many serious books, that will describe these three groups as one.
i'd say, look for books on general prehistoric life, they will always have a few chapters about these creatures, or ask around at university bookstores.
"sea dinosaurs" will get you nowhere, cus technically, nothing of the kind ever existed 
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''In the world I see - you are stalking elk through the damp canyon forests around the ruins of Rockefeller Center.''