I haave a nikon D3200 and want to get an ND filter to experiment with long shutter for the ocean and waterfalls to have that dreamy look. I have the kit lens 18 -55 and a 50mm1.8g Nikon prime. Which should i buy the filter for.? The prime is better lens i think but i will not be able to change focal length
The second part of your last sentence is your answer. ;)
Besides, 18-55 provides you wide angle of view and that is something you will need photographing ocean.
Alternatively you can check Lee or Cokin filters. You buy one glass/filter and you can use it with many different lenses (having dedicated filter holders).