This ramp does go under water on big tides! This photograph was taken during a 4m tide that Townsville experienced earlier this year. But most of the ramps went under on this tide. I'm not sure just how big a tide needs to be for here, but it's safe on 99% of the tides for the year. The road in also goes under on big tides and during flooding.
The other ramp that access the Haughton river is located at Cungulla. Its more for smaller boats, but gets you in closer to the mouth. See here for details.