Turtle shouldn't be the "boss" fish, as it is easy to get. Who could be - is the monster fish, which only swims in deepest depths.
Also, i couldn't resist to purchase the Azure Lemon palette, but didn't know that i won't find any more fish afterwards to purchase the UltraGreat Rod, so i was basically stuck with current level of fishing equipment )
Boss?As far as I know, the turtle is just another catch.