I love it for shiny hunting, I wish more water Pokémon were available by fishing. Such as Lotad, Marill and Finneon.
My longest fishing chain without a shiny was a chain of 201 that I broke purposely. Looking for a Horsea though Skrelp was also available.
My shortest fishing chains include Magikarp at 7, Tentacool at 9 and Relicanth at 12. My longest chain with a shiny was Remoraid at 105, followed by Corsola at 97.
Though it's a nightmare when you keep getting the same crap over and over again looking for a shiny, like Magikarp when you're looking for a Tentacool or Poliwhirl and Basculin with Dratini.
Since I think the chain breaks when you find a shiny it comes down to number of chains/shinies found rather then number of encounters. Like shiny Dratini costs 20 shiny Poliwhirl's. Or 20,000 poke dollars depending on how many net, quick or dusk balls you waste trying to catch them.