You are correct that there aren't any segments of Water on Route 8 that are enclosed by 3 "walls", but there are angle-corners where you can chain fish (I did it all the time when I was playing Pokémon X).
Other places you can chain fish on Route 8 are on 2 square shallow islands (one of them is where you find TM19: Roost), on a shallow island in the shape of this symbol ˹, and on another shallow island in the shape of this symbol ˺. I'd suggest chain fishing for your Shiny Starmie/Staryu on one of those 2 square shallow islands (or in one of the many angle-corners that line Route 8).
Here is a link of Route 8 in Pokémon X.