Jennifer Maas TV Business Writer The recent “Shark Week” tradition of having a DC Universe superhero host the beloved Discovery Channel event continues this year when “Peacemaker’s” John Cena kicks off the festivities this Sunday. Cena’s gig as emcee comes after “Aquaman” Jason Momoa’s turn last year, and “Black Adam” star Dwayne “The Rock” Johnson’s two-year stint before that.