There are no names of underwater dinosaurs because there were no underwater dinosaurs. Sea reptiles that lived at the same time as the dinosaurs were around. Names of these sea-living reptiles include: Liopleurodon, Opthalmosaurus and Tylosaurus.

There is currently no evidence of grass in the Triassic Period.

Until recently it was thought that grass evolved around 55 million years ago but recent findings suggesting that grass became common 66-65 million years ago comes from the very end of the Cretaceous Period. Some dinosaur coprolites show material that appears to have been grass.

It is now believed that Grass's evolved sometime in the late Cretaceous and became widespread just prior to the K-T Extinction Event.

The is no such thing as a sea dinosaur!
They are known as marine reptiles
But if your asking for the biggest it is Shonisaurus the giant icthyosaur or 'Fish reptile'
The largest plesiosaur is Elasmosaurus
The largest Pliosaur is Kronosaurus
Shonisaurus grew up to 15m long
Elasmosaurus grew up to 14m long
Kronosaurus grew up to 14m long
The largest sea mammal was basilosaurus mistakenly called 'king reptile' It was actually a whale growing up to 15-23m long

Edit: People often mistake plesiosaurs and pliosaurs as being dinosaurs. They are not.

There are however dinosaurs that have semi aquatic lifestiles.

Birds are a type of theropod dinosaur, so that makes Penguins ''sea dinosaurs''.

There are also the extint hesperornithiformes that might of had a similar lifestyle.