Automatically reequips your Underlight Angler into your Fishing Profession Tool Slot when you start swimming, and when you leave combat while swimming, so you can immediately gain the benefits of the Underlight Angler without having to do anything.
To work, you must:
- already have Underlight Angler equipped in your Fishing Profession Tool Slot
- have space in your bags
- not be in combat when you enter water (though it can handle you leaving combat while in water)