Look at the weather underground sea surface temperature. Water temp varies with season. Expect a drop of 3-4 degrees Celcius in the thermocline.
Bali and Manado get can realy cool at certain time of the year, I had 21* celcius before in Manado and around it.
For example now in the Indian Ocean side of Java Island in Indonesia, ( Bali share the same Ocean too ) we are getting surface temp at 28 but at 70 feet and deeper it start to get about 24 Cel.
Just watch the kind of water u dive, cool water is now coming into the equator. The deeper the Ocean, the more likely you get cooler water if at the right time. Wherever the location is if it is called a sea and not Ocean, u might not get very cool water in Asia if the spot is still about 5-7 degrees south or north of the Equator.
The El nino might spoilt some pattern. A Polartec hood is very useful if u want to travel light.
Have fun