Image: Wikipedia
According to MarineBio.org "marine biology could have started in 1200 BC when the Phoenicians began ocean voyages." However it wasn't until Aristotle in 384-322 BC made observations and species were identified. Modern day marine biology wasn't started until James Cook mapped unknown waters and described unknown plants and organisms that he saw. Since marine biology is so diverse scientists usually only pick one area such as fishery or rehabilitation or many more. According to What Does a Marine Biologist Do? marine biologists don't handle marine mammals.
In the future I believe that we will be able to go deeper and discover more species of fish. Maybe even discover fish that were once believed to be extinct. Like the prehistoric shark found in Asia that died while trying to observe it, even though they were trying to keep it alive.
Image: Google Images
Video of this here Rare sight of ancient shark | Video | Reuters.com.