The immediate effect of removing an apex predator is a rebound in the numbers of the immediate prey of that predator, and a depletion of whatever that former prey eats. Of course, ecosystems are mostly about equilibria, so eventually, a new apex predator (or predators) will fill the niche of the removed one, and a new ecological equilibrium will be established. Or, if there is no replacement, a different equilibrium will be established. Many introduced, invasive species become so because they find conditions suitable for their species propagation, a lack of predators, an abundance of nutrients, or a combinations of these. In these cases, they may not form an equilibrium for years, instead they may keep spreading until they run into limiting conditions. Look up Kudzu as a plant example (very familiar to anyone who lives in the South). In the aquatic environment, look up zebra mussels (they are a problem in freshwater habitats). One chief problem that introduces species to new environments (with the potential for invasive propagation) is the ballast tanks of ships. If they fill the tanks with seawater in one place, then expel them in another, all organismsm that survive the trip are given the gift of immediate dispersal.
Basically, though, if you are going to kill off an apex predator, you should know about that animal's food webs/food chains, and either model or predict (or make up in the case of fiction) the ramifications down through the food web/chain. The world's oceans represent such wonderful buffers to all kinds of perturbations, however, it would have to be a drastic or long-term ocurrence (even slowly developing but persistent--eventually reaching a critical threshold) to produce immediate or lasting alterations that would impact humans. So, if you can couple your predator removal with some other oceanographic threshold event (like seismic events), you'd be able to create a castastrophic result.
And, as a point of interest, the show Surface was filmed in Wilmington (since there is a large film industry here, they call it Hollywood East), and in fact, a couple of the scenes of the show were filmed in my research laboratory. For example, the scene when the little know-it-all kid talks about hagfish (or whatever they were talking about) as a "primitive vertebrate that feeds on carrion," that scene was filmed in my lab. I still have the 3x5 cheat-sheet card the kid used to remember that line posted in my lab, near where they taped it for him to see. A later microscope scene was also filmed in my other lab room. Several other scenes were filmed here at the Center for Marine Science.