In bright daylight, there’s very little substitute for reducing the light before it reaches the sensor. If you want true long-exposure blur of flowing water, an ND filter is the standard solution.

A circular polarizer helps a bit, but often not nearly enough. Closing down to f/8 and staying at ISO 100 also may still leave shutter speeds too fast, and stopping down further wouldn’t be ideal anyway because diffraction can reduce sharpness.

If you don’t want to fully mount a filter each time, one practical tip is to simply hold an ND filter in front of the lens for tripod shots.

The main alternative is software averaging: take a series of frames and blend/average them later in software such as Photoshop or GIMP. This can imitate the look of a longer exposure for moving water if the shots are aligned well. It won’t be identical to a real long exposure, but it can work surprisingly well when you don’t have an ND filter.

So: for the best result, use an ND filter; for a workaround, shoot multiple aligned frames and average them in post.