The fishbowl helmet is obtained from Murphy during the Freeing Pirate Pete subquest of the Recipe for Disaster quest in exchange for a fishbowl. After the quest it can be obtained for free. It is used, along with the diving apparatus, to dive underwater to the Mogre Camp.
If you lose the Fishbowl helmet, Murphy will give you a new one if you choose the diving option when talking to him.
Subquest of Recipe for Disaster