Indonesia has a tropical climate characterized by heavy rainfall, high humidity, high temperature, and low winds. This means that temperatures tend to be very similar year-round: hot in the lowlands, cooler or (only at night) even downright cold as you move higher up in the mountains.
The wet season is from November to March and dry season is from April to October. Average year-round temperature is between 27 and 31 degrees. Water temperature averages around 26-27°C.
The only part of Indonesia that doesn’t follow this pattern is Maluku, and a few neighbouring areas of Sulawesi and Papua. Most of Papua has a fairly uniform climate year-round: hot, humid and rainy in the lowlands, cooler or even downright cold (at night) but still humid and possibly even more rainy in the mountains. The area around Merauke is the only one where this difference is really marked though.
Best time to go
The best time to go diving in Indonesia is between April until June and October for most diving spots in Indonesia. For Papua November-December are the best months.