My wife and I were in Bali almost 2 years ago, around this time of the year. Surprisingly, the water was not as cold as we had expected... about 27-28C. Then again, our max depth was only about 20m. We stayed on Nusa Lembongan, about a 45 minute boat ride from the main island. We went diving and snorkelling in between Nusa Penida and Nusa Lembongan. Some good diving and snorkelling spots there.
Depending on the dive location, up-currents and thermoclines, the water temp could potentially drop into the low 20's. That's what I've read, but did not experience it personally.
Have a great time in Bali!