Maris, having used Kodak HIE extensively, along with a dedicated IR 720 filter, have you tried this method to gauge IR activity?
Holding the filter in my hands and blocking as much extraneous light as possible, I quickly held the filter up to a single eye. A very quick scan of the desired image one is considering, then pull it off, wait a second then have another scan, and so on.
Using this method I was able to quite confidently know firstly if there was much IR light around, and also if the desired subject was glowing in a manner I was hoping for. As well as the best direction for the best effect; which I'm guessing you already have a handle on.
Water goes black, as does high sky, just like in your picture. What I was usually looking for, was glowing green foliage and using that method worked wonderfully.
I also never used a light meter, I just had three exposure conditions and three shutter/aperture variables and found this worked perfectly as I was only really interested in the IR part of the spectrum.
Bright sunny/ cloudy bright / sort of cloudy bright. If any of these conditions weren't met, then IR in general wasn't going to work too well.
If you had some Tech Pan, you could try one sheet with the 720 filter and one with the 680 filter. there should be a noticeable difference I would think.