whats the argument that supports it? he was a bit of an idiot and couldnt put it forward.
Plenty that might be reasonably considered. The most obvious being that only certain entities are morally considerable. For a carnivore, that category might exclude all non-human life, a vegan might deem all animals morally considerable and therefore avoid products which derive from (or perhaps instrumentalise) animals (yes, a fish is an animal).
A lot of people sit somewhere in the middle, including your "aquatarian" or whatever label he gave it fella(pescatarian is normal word for this). It may well be that he considers fish/seafood morally less considerable than mammals/birds/amphibians/reptiles etc... for any number of reasons. Most common (and closely related) ones might be that 1) Sentience confers moral consideration (less sentient beings (ie molluscs, crustaceans...) less morally considerable therefore can be eaten). Or 2) Only mammals/birds (etc) feel pain, therefore if the avoidance of the causation of suffering is the basis for decision making, pescatarianism may seem justified.
Argument 2 I would argue falls down in light of evidence that pain can be felt even in lower order animals, and also that I personally do not take pleasure/pain to be justifiable as a singular basis for ethical consideration. A lot of people might still articulate this type of argument as justification.
Argument 1 is a little tighter, but only if you believe that sentience is a reasonable basis for establishing moral considerability (and you can argue why you draw the line where you draw it). Again, I'd argue that this is not satisfactory.
There are loads of other more complex arguments based in rights and virtue ethics, but the two above are the most common ones that I've heard.