First, this is just speculation! But based on the use of (mostly) actual fish species by Blizzard for the new MoP fish, I think it is possible to use this information to guess whether these fish will be caught in Inland or Coastal waters as all previous fish have been divided in this way.
Inland Waters (freshwater species)
Golden Carp - First caught. (common catch)
Jade Lungfish - First caught.
Jewel Danio - First caught.
Krasarang Paddlefish - First caught.
Redbelly Mandarin - First caught.
Tiger Gourami - First caught.
Coastal Waters (seawater species)
?? (common catch)
Giant Mantis Shrimp - Expected to be caught by fishing, not yet available.
Reef Octopus - Expected to be caught by fishing, not yet available.
Emperor Salmon - First caught. (live in the sea but migrate to rivers and used as such before in game)
Spinefish - Expected to be caught by fishing, not yet available. (to balance things!)
There are the three quest fish which can be regarded separately and found where their normal relatives live.
Flying Tiger Gourami - Expected to be caught by fishing, not yet available.
Mimic Octopus - Expected to be caught by fishing, not yet available.
Spinefish Alpha - Expected to be caught by fishing, not yet available.
The reason I have put ??? for coastal common fish just think Cataclysm and Murglesnout/Sharptooth. I just don't see them having carp in seawater!
The split then is a little uneven right now so perhaps the two remaining uncommon fish will go in the sea to balance things. Not really sure if any of these could be called 'common', so it makes more sense they add one for coastal waters.
I am sure it will work out a little differently, or totally if they go for zone wide fish (I hope not). But it's good fun doing the detective work on the possibilities and these makes sense from what there is to date.
For those who do not have beta access, currently all fish can be caught in both inland and/or coastal waters in each zone. The only zones where this is not currently the case is where a zone has no coastal waters (Vale of Eternal Blossoms for instance)
I think that they will divide it as they have done historically during the beta for two main reasons. They always have done it this way, which adds nuance to each zone, helping to provide individual zone identity/interest.
Secondly, unless they have no plans to continue with both the Limnologist and Oceanographer achievements, then they will need to separate the required fish (not yet added to the achievements yet) for these; i.e. freshwater and saltwater species respectively.
Now some way into the beta with game confirmed catches and datamined information (fish in recipes etc.) we have a reasonable idea of MoP fish with perhaps two remaining unidentified ones yet to be named (shown as 'Uncommon Fish 3 and 7' on Wowhead)
Having seen the icons which El added here plus four more found by Wowhead yesterday, there are now 12 in total. This tallies with the 10 known plus 2 unknown. it can change and probably will, with other minor catches such as a herb (think Azshara's Veil), rare catches. But here is how I see it breaking down;
This is based on checking the actual habitat of the fish species via wikipedia. So for instance, Paddlefish live in freshwater if you check the link to Wikipedia. The only exception so far are Spinefish which have no obvious comparison so are probably 'fictitious fish, swimming in a fictitious world...'
p.s. As a reflection of the detail to which Blizzard go in choosing and designing the fish, I read on Wikipedia about danio fish (ref. Jewelled Danio) that;
The name "danio" comes from the Bengali name dhani, meaning "of the rice field".
From what I have seen in Pandaria so far, it reflects the attention to detail evident throughout.