During the quest Beneath Cursed Tides, Wizard Myrtle casts a spell upon you, enabling you to breathe while you dive down to investigate the fate of Tutorial Island and its tutors. That spell lasts beyond the end of the quest, since you may dive down to do the fishing and farming D&Ds without even having to talk with Myrtle - UNLESS it's just presumed she casts the spell on you each time you dive, without the need to speak with her. I'm just throwing out a theory here. I have no evidence to prove or disprove Myrtle casts a new spell on you before you dive down for the fishing/farming XP. I only know you can dive without having to talk with her.