I'd also like to mention that the stamps on the sides of the Naniwa Choseras state that the hones should not be left soaking in water for long periods of time (i.e. more than 1 hr.) because the hone could deform.
The Gokumyo, on the other hand states that no mater how long you soak it, it will never deform. This is one you can leave in a honing pond even in between honing sessions.