The quickest way to level up fishing is to cast your line as far as you can, then reel it in to around 16m. If you see a larger fish is approaching your bobber reel you line in to 9m or less. You gain experience while reeling in your line, this is why I suggest to maximize your initial cast. Recently I have been fishing while standing on the very edge of my boat after giving it one quick row to set it in slow motion and have been very successful. If I lose a fish due to low stamina, I drop my line in with 5-7m of line out and 9 out of 10 times I am able to catch it without using the assistance of a stamina potion.