Indian and Pacific Ocean waters mix here, bathing Nusa Penida in currents that can be strong and unpredictable. Sites on south and east Nusa Penida are rarely visited because of their unpredictability. Water temperatures are cool, 21º-24ºC (72-78ºF), and can drop as low as 15ºC (60ºF). Drift diving is the norm and visibility is 20m (66ft).

Nusa Penida's best feature is its pelagic marine life. Nutrient-rich upwellings attract eagle rays, sharks, tuna, and mantas. The most famous visitor is the mola-mola and the reason most people make this journey.

The ocean sunfish or mola-mola is an odd fish. It's related to the pufferfish but the body is flattened and looks like a giant head with a small mouth, two long fins and a scalloped back end. Weighing up to 1500kg (over 3000 lbs.) and 3m (10ft) long, the mola-mola often lies on its side at the surface of the water, thus the name sunfish. It feeds on jellyfish, plankton and small crustaceans.

The mola-mola is a pelagic fish but when the water is at its coldest, about 15ºC (60ºF), they appear off northwest Nusa Penida and Lembongan islands. Found at about 30m (100ft), they're often accompanied by an army of smaller fishes that clean parasites from them. Nusa Penida, from July through September, is one of the best places in the world to see mola-molas.