Jeez! Sorry for not having caught that myself, but THANK YOU for the help!
I have done some tests, and am pleased to say that jpype (both JPackage and JClass) now works wonderful and fast! :)
Also, I appear not to have any problem with the jar "dependancies" problem that you mentioned.
Even though I have dependancies on two additional jar files, neither of which I have named to JPype, the data works fine. My main class SteamPoint immediately calls a second jar IAPWSF97 - and the data return is correct.