How long does it take by boat from Sydney to Brisbane?

It would depend on how fast your boat is. The distance is 397 nautical miles, so at 20 mph, it would take 19 hours and 48 minutes. At 30 mph, it would be 13 hours and 12 minutes. At 40 mph, it would be 9 hours and 54 minutes, and so on. Basically, the faster your boat, the shorter the time.