The hottest months are January and February, when air temperatures can vary from the mid-30s Celsius to the low-20s at night. These are also the wettest, most humid months. The coolest months are July and August, when the maximum temperature is usually in the low 30s. The maximum and minimum sea temperatures lag behind the air temperatures by about a month. They vary from the high-20s in February to the mid-20s in August.
The best water visibility is in the cooler months, from June to about October. However, I don't recall visibility ever dropping below about 15 metres at the very lowest.