Believe it or not, you can shoot long exposures even on a bright sunny day. The trick is adding a round neutral density filter to your bag. Sometimes dubbed “black glass”, these dark screw on options block a great deal of light from entering the lens.
For example, if the proper exposure gave you 1/125, f22, ISO 100, the water’s motion would be largely frozen. By adding a nine stop ND filter, you can keep the aperture and ISO the same, but drop the shutter speed all the way down to four seconds. This will create that dreamy effect previously limited to low light situations.