Underwater generally refers to below the surface. You can take a look underwater by visiting the basements of any of the towers of the Noodles Academy of Modern Magic, some of the Quarters in Yalahar, this cave, or the Realm of Dreams. Other than that, there are only a few buildings in Tibia that have their own private aquarium: the Nautic Observer, The Tibianic, the Castle of the Winds, and the White Wave Cellar.
However, there are some underwater areas in Tibia you can actually be in. These include:
- Calassa (here);
- Fiehonja (here);
- The Frozen Trench (here);
- Both Sea Serpent Areas (here and here);
- Yalahar's Sunken Quarter (here);
- The Seacrest Grounds (here);
- The two lower floors of the Quara Grotto (here and here);
- Drefia (here);
- An area of Krailos (here);
- Part of the Dream Realm (here, but only during a quest).
When going underwater, you will need to wear Diving Equipment to prevent the incurrence of 5 times 20 Drown Damage kicking in after four seconds of standing still; the only exception to this rule is Fiehonja, where you can breathe freely with any helmet on. Note, though, that equipment can be removed at any time without having to exit the water. The only areas which actually require Diving Equipment to enter are Calassa, the Frozen Trench, both Sea Serpent Areas, the Sunken Quarter, and Quara Grotto. The equipment can still be taken off once there, however.