For silky water, slow shutter speeds are mandatory, usually at least ½ sec -- and 2 or 4 sec is a lot better -- so a tripod is also necessary. Thus, even with ISO 50 or 64, if the water is lit by any Sun, you'll need a good ND filter. Note that ND beyond 2-stops (or .6) for digital sensors require IR cut as well or else you get an ugly yellow-brown color cast that cannot be corrected in post, so make sure you get ND with IR cut. I like Tiffen and Zeiss/Heliopan, but not sure brand matters much for ND. My reco is 2, 1 at .9 or 3-stops and one at 1.8 or 6 stops. The 3 stop is good for general use in more dimly lit situations like early AM forested streams or pre-sunrise surf, and it remains easy to view through an SLR with. The 6-stop will be required when any Sun is present, but you can still see through it well enough to compose. Both stacked will be required for full Sun waterscapes and you cannot really view well through that, so I leave the 6 mounted for composition, then add the 3 on top before firing as needed to get to my desired shutter speed.
As for wides, my personal favorites are the Sigma ART series, but they are pretty massive designs. But then they work very well, so I suffer them
I think every bag should have a 24 or 21 in it as a super-wide, so I'd start there. If your current wide is a 35, I'd get the 24 to start; if it's a 28, I'd get the 21 to better round your set out.*
*I personally find 21 a bit too wide for most landscapes, while not really wide enough for a hyper-wide** capture, but then like the 21 as my goto super-wide for street/travel since 24 is sometimes too tight for tight city streets. I have not tried it since I already own the 35 and 24 ARTs and the Nikkor 28/1.4, but the Sigma 21-35 ART zoom just might be a handy option if you can live with the size/weight...
**I call anything wider than 20 a hyper-wide, others definitions will vary. In my case, I find the inexpensive all-manual Samyang 14mm a real gem performer -- excellent resolution corner to corner at f5.6 and 8, at only around $300 new. It is good enough I can crop with it, and then I use that wide of range so little I don't feel the need to own anything else under 21...