Chordata is the phylum of the vertebrates, but not all chordates are vertebrates. Vertebrata is the subphylum of chordata. The following are the major marine classes of this subphylum:

Class: Agnatha--hagfish, lampreys

Class: Chondrichthyes (or Elasmobranchia)--sharks, etc.

Class: Osteichthyes--bony fish

Class: Reptilia--sea turtles, alligators, etc.

Class: Mammalia--whales, etc.