I'm betting there's no really exciting giant animal 'hiding' in the ocean. A lot of the species we haven't found yet are microbes and other small plants and animals. There will be exciting things to learn from these, especially ones that live in hostile environments that are too hot, too acidic, or have too much oil, and so forth. Another environment we're studying is the very cold water in lakes under the ice in Antarctica.

I'll predict that the largest new animal found in the oceans is not longer than 2 ft. But maybe there will be longer new animals found if you count things like a new kind of shark that's related to sharks we already know about.

The chance of finding a megalodon or some huge prehistoric creature hiding in the waters of the oceans is very small (but not zero!). There are parts of the ocean that remain poorly explored, especially parts of the deep sea, but what you say about not having discovered "half of the ocean" is not true. We have explored the entire ocean in very cursory detail (e.g. we have maps of the seafloor), and very little of it in great detail. Large animals like the megalodon are much easier to find than smaller animals. It is much more likely that smaller "prehistoric" creatures do still dwell in our oceans though, and that we have not found them yet.