There are two ways to fish on the islands. One is with a spear (either in the ocean or pond) and the other is with a fishing rod in a pond. After you have completed the initial guided game, you will have the ability to create your own fishing pond with the building tools option. The pond does not need to be big in order to be useful for your characters.
This first part of two fishing sections is for spear fishing. The more you have your character spear fish, the better he/she will become at getting more expensive fish without spearing him/herself in the foot.
Spear fishing is a solitary activity for the most part. One aspect of spear fishing, if you can find a spot deep enough that your Sim is standing in waist high water while spearing fish, he/she will also handle any bladder and hygiene needs at the same time.