The dry season (May to October) is the best time to visit. Roads are passable, the sea is calm enough for boat crossings, and visibility underwater is excellent. August and October are the peak months for mola mola sightings; manta rays are present year-round but more reliably encountered from April through October.
The wet season (November to March) brings rough seas and rain that can make the clifftop viewpoints slippery and dangerous. Crossings from Sanur are occasionally cancelled. The island sees far fewer tourists in this period — prices drop and the viewpoints are almost empty, but planning around the weather is essential.
Kelingking and the western viewpoints are best visited early in the morning, both to beat the crowds and to have the light behind you for photos.