Unlike most accessories, informational accessories such as the Fish Finder do not need to be equipped in an accessory slot. The Fish Finder may be carried in the player's inventory without loss of functionality, unless equipped to a social slot. Additionally, the info it provides will be shared with nearby players of the same team in multiplayer. Keep in mind modifiers will no longer apply to the player when not equipped.
On Desktop version, turning off autosave can be used to force getting a particular item without having to play through multiple quests.
To use this for a fishing quest, set up the character with the quest fish in the inventory, and the Angler near the spawn point. Then Save & Exit the world, rejoin, and turn off autosave.
Turn in the quest, and if the quest reward does not contain the desired item, exit the game without saving. This can be accomplished by ending ("kill") the game program "from outside", rather than with its own "Save & Exit" option. In current versions, the window-close button will be grayed out, but the Task Manager or Alt+F4 can still be used to kill the program.
Now begin the loop: Turn in the quest, and if the desired item is not rewarded, exit Terraria without saving and rejoin, then turn in the quest fish again. Repeat as needed until the desired item is got.
If the current quest has a guaranteed reward (e.g., the 5th quest), it is only possible to get multiple copies of that item, instead of a random reward. Just save the game after the quest is awarded, play through to the next quest and try again.
The item closely resembles the Dragon Radar from the Japanese media franchise Dragon Ball.