with the fisheye, impossible to use a screw-in polarizing filter, you need the rail system (which I don't have).Fantastic! The location looks perfekt for a landscape shooting. I would maybe use a ND-filter to get smooth water and clouds. Maybe I'll able to see it until I'll get too old to travel.
On the other hand, I have several long exposures(ther 10 s.)
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with an ND400 + sigma 10/20 filter ... obviously the field is less wide (just mountain + 1 single fall) ... here is an example!