All of the shapton stones require no soaking. Same with the naturals.
Here is a good set to use that comes with flattener and holder:http://www.chefknivestogo.com/sh6pcstset.html
You could add the 500 and the 8K to this set too but these 2 stones will do the job.
Also, as you mentioned the Naniwa superstone line is also splash and go. They are softer than some of the other stones we sell but they work well. I tend to like the higher grit superstones more than the lower grit ones.