What was one of the biggest dinosaurs?

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.