I was on a northern liveaboard safari from 8-16th December last year and the water temperature was around 23-24°C. You are further south, so it might be a degree or so warmer, but I would imagine it will be around that sort of temperature.
On the surface, it was high 20s, even low-30s during the day, but there was a bracing wind which meant it felt bloody cold when you were in the RIB on the way back to the main boat. I used a 5mm semi-dry with a 2mm vest under and I was plenty warm in the water, but got the shivers a bit towards the end of the week when getting out of the water and into that wind.
Hope this helps a bit
Mark