Ocean Science is a free iPad app that at least six people have recommended to me over the last few weeks. The app is essentially a seven chapter, multimedia ebook about the basics of ocean science. Each chapter includes animations, pictures, video, and text. You can toggle back and forth between the multimedia elements and the text in a document. A useful aspect of the app for students is that you can take notes within the margins of the text.
The chapters of Ocean Science cover aspects of biology, ocean currents, conservation, and exploration. Ocean Science could be a good app to use as part of an introductory marine biology lesson.