Are sharks related to sea dinosaurs?

No. First of all, there are no "sea dinosaurs." During the time of the dinosaurs there were marine reptiles such as plesiOSaurs and icthyosaurs, but they were not closely related to dinosaurs. Sharks are cartilaginous fish. Reptiles are ultimately descended from bony fish, which split off from cartilaginous fish more than 100 million years before the first reptiles appeared.