Follow these steps to wear the Fierce Diety mask anywere.

1. Go to a pool of water that you can walk into, (like the one in Termina Field).
2. Equip both the Zora mask and the Fierce Deity mask.
3. Put on the Zora mask and walk in the water so the water is up to Link's waist.
4. Take of the mask.
5. Dive down until Link's hand touches the bottom.
6. Quickly put on the Zora mask.
7. Now make Zora Link try to swim to shore. But he won't move(!)
8. If done right, Link should do a roll attack and then the B button should be disabled.
9. Put the Fierce Diety mask were the Zora mask is, (don't take it off).
10. You should be able to put the Fierce Diety mask on.

Note: To take it off, swim in some water, and you'll be able to take it off.