The ocean can rightly be considered the final frontier. There is so little we know about the depths of the ocean, even after attempted probes to the region. This indicates that there probably are creatures we haven't even discovered yet that lurk in the watery depths, just waiting to be found.
With 95 percent of the ocean still being unexplored, scientists say that probably 2/3rds of of all sea creatures are still undiscovered. Some of this is because parts of the oceans are so inhospitable or so deep that they cannot ever be explored sufficiently. For example, we will never know what lives at the bottom of the Marianas trench, because no vehicle can withstand the pressure to reach the bottom without being crushed.
There is a great deal about the deep sea that we have not explored yet. The more we explore the more likely we are going to find new things and new creatures. The sea is vast and deep, and it needs to be explored. There are likely to be many new creatures.
There could absolutely be creatures deep in the sea that we do not yet know about. Our knowledge of this planet is still very limited, and we are making discoveries and learning new things every single day. There are many corners and depths of the planet we have yet to fully explore, and what we find there when we do could definitely surprise us.