What is the ideal shutter speed to turn gushing water into a dreamy blur? Half to one second would be a good starting point. There is no hard rule as the outcome or creamy effect will depend on the speed of the flow and the angle.
It can be slower but you would probably need an ND or polarizing filter to cut down the light. If you don’t have those filters, go later in the day when sunlight is less harsh or find a shady spot. A tripod will come in handy but you can also place the camera on a rock. Trigger the camera with the 2-sec self timer to eliminate shake.
The effect works better if the waterfall, river or stream is cascading. You don’t need a big waterfall to experiment, though. This was from a small stream in Hulu Langat.