I am going to Australia in mid-August and want to go to the Great Barrier Reef, Gold Coast, and Brisbane. I wonder if the weather at that time will allow me to go into the water?

No problem getting into the water. The Great Barrier Reef is close to the equator and has very high temperatures all year round. I went there in mid-August last year and even did some diving. Just wear shorts and shorts when going out. The temperature on the Gold Coast is quite high during the day, so you can swim in the water (even small children can do it). But it gets cooler at night, so wear long-sleeved shirts and long pants. In short, go and have fun with peace of mind.