option 1: avoid filters by shooting during the burn, like 15 minutes after sunset, best time to shoot IMO
option 2: look into a LEE style filter setup with 100x100mm filters. I personally ditched "LEE" brand and experimenting "Bombo" holders, very inexpensive, although, my personal setup is even more expensive than LEE since it works on the CV10. Regular adapter ring goes for only $7. Worth exploring if you have multiple lenses and very fast in the field, sunsets dont last very long!
option 3: Circular filter of your largest lens with adapter rings for smaller stuff. 82mm filter + 77 to 82 adapter
golden hour, 3-6 stop gives you best water texture IMO, the wider the lens generally longer exposure time you'll want (1/4-2s)
6 stop probably my favorite for that type of shooting, just enough, not overkill. 3 stop can work too, 10, way overkill.... that said, 3,6, 10 are my core filters. I have a square CPL, but dont use too often. I dont use grads, just bracketing. You'll have to find a good exposure for the water flow, then grab a few brackets for the sky. You eventually get better at post processing and it becomes second nature.
LEE brand filters this day and age.... overpriced. you get what you pay for does not apply, in fact my LEE's have performed worse than some of my generic stuff!
haida makes the best 10 stopper, but a tossup now with cheapo ICE filters*
*older ICE filters were really bad or possibly fakes, so buy with a good return policy, 10 stoppers are the most tricky due to high density
zomei/ice have good 3/6 stop filters, generally very reliable, and made from glass (LEE plastic too easy to scratch).
I do have a complete set of 82mm filters that I've been thinking of selling. A combination of the filters that give the cleanest results for smallest price. The CPL costs more than all the filters combined 
attached is my newish filter system by bombo, which has various adapters from inexpensive to very expensive. The ultra small size has made a difference for me, I can fit in my photo vest just barely, couldnt do that with the LEE.
I forget the option 4: if you have a fairly burly tripod, you can simply hold the square filter on the lens itself and take a shot. not the best option, but I've done it before. I think I lost an adapter ring in the sand, somehow fell off the lens
Image hosted by forum (
862236)
© Charlie [SHARE LINK] THIS IS A LOW QUALITY PREVIEW. Please log in to see the good quality stuff. Sony A7siii/A7iv/ZV-1 - FE 24/1.4 - SY 24/2.8 - FE 35/2.8 - FE 50/1.8 - FE 85/1.8 - F 600/5.6 - CZ 100-300 - Tamron 17-28/2.8 - 28-75/2.8 - 28-200 RXD
Panasonic GH6 - Laowa 7.5/2 - PL 15/1.7 - P 42.5/1.8 - OM 75/1.8 - PL 10-25/1.7 - P 12-32 - P 14-140