Silky sea

I popped in to Sheringham this morning and thought I'd take the opportunity to take some photos of the tide coming in. I took most of the photos with a fast shutter speed as I hadn't taken a tripod, but took a few on a slower shutter speed to try to get a silky sea as above. I preferred the silky effect. Obviously the longer the exposure the more light that gets in, so could anyone out there suggest a suitable filter to prevent too much light? (I know I can adjust it in post processing with the gradient filter, but would quite like to get it right in camera).