How fast is that water moving? You may not need many minutes worth of exposure for that. Oftentimes, I find anything over half a second will be more than enough. Might even be too much.
My recommendation would have been Pan F+. It's worked well for me in the past.
If you really have a very long time requirement, I would recommend in 35mm EDUPE; it's a color slide film, unfortunately not available in 120mm. It is built for duplicating, is extremely slow, ISO 12-ish, and has a pretty good tolerance for the longer exposures. Kodak says it's good for quite awhile; I'd have to check the tech specs, but off the top of my head, I think I remember it being about 10X longer than what I would have expected; it will accept a much longer exposure before a reciprocity factor kicks in.
I know it's outside of what you were asking for; wrong size and process; but, it has such a great tolerance for long exposures, that your desire to keep the shutter open for so long reminded me of it. J.