I got the Nisi ND filter. It's literally just a huge plate of glass. I got the Haida 150 series holder for the Tamron and it has a screw fastener that I just didn't put on at the time. So as I rotated the holder, the actual mount just fell out and the filter fell and shattered. I was about to get a lower profile shot as these long exposure shots look better from a low perspective but the inevitable happened and right after my girl told me to watch out and not drop it. Insta karma at its finest.
Amazon product ASIN B016Q4GEAI
I must say though, this type of photography is a real chore. To setup all this stuff, adda a trigger, and then calculate the time via an app is just so time consuming rather than just shooting it and calling it a day. Granted I am happy I can even get shots like that but there is just way too much prep that I am not used to.
I still need to shoot during sun up or sun down with the ND filter, You can get literally minutes to a shot that adds to the smoothness of your ocean shots.
Here are some examples:
https://iso.500px.com/long-exposure-seascape-tips/