filter question

I am hoping to try and get some different photos this year.  I take lots when I am out and about and I am happy enough with them as they capture what I see.  What I would like to do is try and experiment a bit this year and from my little understand a different filter may help with this.

What I am hoping to achieve:

I enjoy using slow shutter speeds on waterfalls and little streams but finding it quite difficult to achieve this in bright condtions, it is fine when I am sheltered by trees and i can use a slow shutter speed without the image becoming over exposed.

I see lots of beautiful caostal shots where the ocean has a lovely creamy effect, particularly in the evening/sunset.  I am hoping to visit some of the Scottish coastline this year and would love to play around and try and get something like that.

Am I right in thinking that a ND filter might be of use to help with such shots?  I have been reading about nd and nd grad filters and it gets a little confusing.