The answer to this question is contingent on your definition of strong. This is a fairly ambiguous term that can be applied to a variety of species in a variety of situations.
Consider the following scenario:
Of all the sea animals, the Saltwater Crocodile has the strongest bite force.
Bull sharks are known to have the most powerful bite of any shark.
The Blue Whale is one of the world's largest and most powerful vertebrate species.
On the other hand, there are numerous lists on the internet of the "strongest" species in the waters.