On Wed, 23 Oct 1996, Ben Kosse wrote:

My question is, why didn't Ranma et. al. just jump in the right pool
after they got cursed?  Especially since it appears the Guide had a map
of the pools?  For those who say "He made it after Ranma left,"  I
counter that the Guide always knew the name of the pool people fell into
*before* they emerged, so he must have all the pools memorized.

     No one asked, and obviously, if the guide said the curse was
permanent, then he didn't know the curse could be cured by the appropriate
pool. At least that's a rather simple theory, based on only the anime, as
I do not have the manga.

Best answer I got: They didn't figure it out until after they were in
Japan.