The Legend of Zelda Phantom Hourglass Walkthrough part 14

The last part of the fishing sidequest.
You first need to catch a Rusty Swordfish before you can catch the Neptoona.
Neptoona can only be found in a spot marked with a icon in the shape of a swordfish.

The only thing you get for a Stowfish... is a ship part... that's all...
You can sometimes find a Stowfish on a Toona, a Loovar, a Rusty Swordfish and if you're really lucky even on Neptoona. Chances you find one are really small, though and just not worth it...