This is salmon fishing on the Tweed at Berwick, using proper salmon cobles, just like it says in the books!
Watching how the two cobles we saw were being used, the wide bouyant stern and flat deck aft are clearly developed for carrying and paying out the net, which hangs in the water across the river. This is what the photo clearly shows. Once the net is across the river, the net is hauled in from the bank.
In the Tweed, the boats are pushed by people in waders as well as rowed, which I guess limits their size.
I guess two other design issues are that the boat should row well (maintaining its direction between strokes) despite that wide stern, and also cope with the short waves you see in shallow estuaries.
All of that explains why these boats have evolved over centuries to be this very particular knifing now, pronounced sheer and bouyant stern shape.
For more, see this article on salmon cobles by Mike Smylie. https://classicsailor.com/2018/04/scottish-salmon-cobles-smylies-boats/
Re your note on Berwickshire salmon cobles, they do not have a ‘knifing’ bow like the English cobles further south and are more a rowing than a sailing boat. In shape they are rather like a wider and flatter curragh but with a wide transom stern to take the weight of the net.
They have sharp bows, which I think is a fair observation. They clearly don’t have the deep, narrow fore sections of cobles further down the coast, but I didn’t suggest that…