Could the ocean hide unknown intelligent life?

Yes — the ocean could still hide unknown intelligent life, especially animal intelligence we haven’t studied yet. Deep-sea habitats are hard to explore, and we keep finding new species and unusual behaviors.

That said, a hidden technological civilization is considered extremely unlikely. Underwater life faces major constraints on available energy and resources because sunlight does not reach most of the deep ocean, so photosynthesis is limited to surface waters. Also, fire-based industry and many forms of metallurgy are very difficult to develop underwater. In addition, a large advanced civilization would likely leave chemical, acoustic, or physical traces that ocean science and seabed mapping could eventually notice.

So the best-supported answer is: unknown intelligent animal life is plausible; a secret advanced underwater society is not impossible in a strict logical sense, but it is extraordinarily unlikely.