Beach of Scathach - underwater inhabitants

I'm not sure what you mean by "demarcation line".

So far, I've only gotten king crab, golden scale fish and deep-sea fish from the ocean(south/troll) area, and I've only gotten sweetfish and taitinn carp from the river(north/sahagin) area. I've gotten flying fish from both. I've stopped fishing at the north because the south is much safer(slower aggro, slower movement, less frequent attacks, no f***ing teleport, and they only use melee; whereas river rangers seem to triple aggro instantly, and spam teleport and high ranked lightning if i try to run).

Also, during the costume/homestead fishing event, I stayed there for hours and didn't get a single spawn with the event bait.(more evidence that it's part of the fish list rather than a feature of the map)