.1 - .75 second water has line and texure. use 30sec water will totally foggy. Slow water need more time to get flowy effect, fast water needs less time to get flowy effect.
if after ND, it's too dark to focus. Use manual focus. Use 1000 ND (10 steps), if base if 1/30, then use 1/30 x 1000(ND factor)= 33 second. 1/30 - 1/15-1/8-1/4-1/2-1-2-4-8-16-30 that's 10 steps.