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You didn't mention it, but I'm going to assume that you are using a tripod if you are shooting with a shutter speed of 2 - 8 seconds. The problem you are experiencing is due to lens diffraction which occurs at very small aperture settings. Here's a link to a brief explanation in case you're not familiar with the term: http://www.luminous-landscape.com/tutor ... tion.shtml
A polarizing filter can be better than an ND filter if you are getting glare from the sun on the water. In most cases, the ND filter will work better.
Hope this helps.