I used Ektar 25 from early summer 1999 just into the new year of 1999 actually. It was a great film, but exposure had to be on the money.
Later on Ektar 25 Professional came out, I used that almost exclusively until the late nineties, exposure for that film also had to be on the money.
Generally, I found the true speed of Ektar 25 Professional to be 15 DIN or 25 ASA. Sometimes I would find a slight overexposure to be a good thing, exposing at 14 DIN or 1/3 of a stop slower than 25 ASA was sometimes right on the money.
This is a film that really requires you to be the one that processes and prints, otherwise quite a lot of your efforts will go unnoticed.
The film has a staggering reciprocity characteristic of about 10 seconds, before you have to allow longer exposure. This is an extremely good feature in low light stuff you may be shooting.
By all means bracket, but if you can I would suggest you expose, develop and print one roll first. Looks as though you cannot do this, but as this is obviously a film in short supply and can never be replenished in your camera bag, maybe you could think about it.
Be prepared for a high contrast outcome, which in Australian light is not too good, unless your waterfall is shot on an overcast day or it is in a forest.
Mick.