Currently, the only fishing place I've found is on the Deserted Island.

Fishing with a frog instead of a worm seems worthless.

Is there any location where fishing with a frog is worthwhile?

I've unlocked the Deserted Island, Sandy Plains, and Flooded Forest.

use the frog to fish out Gobul, a monster that resides in the flooded forrest

Yep, gobul, and urgent quest at the end of the 3* quests.

And also, the frogs only work on Gobul, so keep 2 or 3 for him. And also, there are fishing spots in the flooded forest, and sandy plains. In SP, in the oasis (area 7) near the sides there are some fish. And in the FF, There is a part of moss and reeds in area 4 or 2 I think that you can fish at.

Yea, the frog is worth it JUST for Gobul. Everytime my group fishes him out, things go waaay smoother. He's not..-tough- underwater, but he is very annoying.

Yup YUp Gobul...you catch him with the frog to get optionnal qwest completed

Fishing Gobul out does 2 things:

1) Lets you get a significant number of hits on him as he flips upside down as he comes out of the water
2) You then will proceed to fight him on land until he runs out of stamina or is hurt too much.

On land he is one of the easiest bosses.

He also always drops a shiny item when fished out, just wait for him to move. Usually a wyvern tear but it can be a few other gobul specific items.