Nessie is native to the loch, which is cold. She needs to stay warm, she needs to be streamlined, and she needs to have flippers, too.
I did a lot of research into seals which are found around the coasts of Scotland. More so than they are found anywhere else in fact, as these pinnipeds are quite rare in other waters.
I was interested in the flippers of a seal. As much as there was some interesting sonar findings which, when enhanced, 'showed' a diamond shaped flipper with a clear spine, I think Nessie's skeleton would have a lot of seal-like qualities. Seals do have very long necks, although you can't tell because they're so fat. I feel like there's definitely Nessie connotations in there, with her big long neck.
It would allow her to move easily on land, too, and the big eye sockets in the skull make it easier for her to see in the really murky waters of Loch Ness.