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+1 @ ND (or polarizer) to lenthen your exposure time.
Given the oblique angle of your lighting from this position, the combination of polarizer and your 1 stop gnd could provide some some dramatic sky + more blurred water ... diggin' the prospect of seeing this one shot with both.
Toss on another stop to f11 and you could be picking up nearly 4 stops (1 @ gnd, 1 @ f11, 2 @ polarizer) total, or 3 stops more than this one. That would get you down to 1/10 sec compared to the 1/80 sec on this one. (or other speed of choice through aperture variation), and you could still angle your gnd.
ISO 50 ... while it sounds good in digital ... is often times just a "false" ISO and is essentially the same as ISO 100, just with an EC applied from what I've noted at DXO's sensor testing of various cameras. This doesn't really gain you anything different than ISO 100 in terms of your actual exposure/shutter relationship. A "true" ISO 50 would of course be a different matter. I shot my Canon's at ISO 50 for a while ... finally realizing I wasn't gaining anything really different.
Speed/motion blur are also relative to FL and subject distance, so some variation there (diff comp) can afford a different degree of blur achieved, even if limited to the same shutter speed. Pretty subjective @ the range from milky white to frozen solid, but it sounds like you're trying to find a little of both, which can be a bit trickier/more precise speed/distance/time relationship than the extremes @ milky or frozen.
GL, looking forward to seeing more.