What they want to understand is what is your approach to that brainteaser. In this case i would suggest in a general perspective to do the following:
Start saying that in each KM3 there are x Number of Fish, then you can proceed by calculating the numer of fish in each ocean, having in mind that there are a lot of differences in the composition of the Atlantic´s ocean than the Dead sea.
Based on that, you should calculate the volume of each ocean and then making multiplying it by the number of fish in each of them. if you want to go further in your analysys, you could come up with the fish born seasonality, which could impress your interviewer but at the same time could mean much difficult calculations.
Hope it works.
Juan Pablo Montoya