An old low head dam on the Cedar River between 6th St. and the railroad bridge provides some fast action at normal water levels. The dam is broken out in the middle and has a fun little pour-over and a long wave. The area should be relatively safe unless you see something obviously unsafe. At flows above 2000 to 2500 cfs http://water.usgs.gov/ia/nwis/uv?05464000 a boil starts to develop on the rest of the dam and the wave gets stronger.
Downtown Waterloo has some very fine eating establishments within 3 to 4 blocks of the play hole.
Find 6th St. in downtown Waterloo and proceed to the N side of the river. Turn right once you cross the bridge and park along the flood wall by the tracks.