When I shoot seascapes I usually use moderate shutter speed - from 1/10-th of the second to 3-4 seconds. Such shutter speed allows you to smooth the waves a bit but also keep texture in the water and show motion and energy of the sea. In most cases I avoid very long shutter speed when shooting the sea, because the result look more like a fog or milkshake and I don't like it.

But in some cases very long exposure can show you very interesting and exciting results. In this photo from Isle of Eigg I used tremendously long exposure of 241 seconds. Even after sunset I had to use ND 10-stop filter to achieve such a long exposure. Although sea waves had blurred into the mist this image still have a lot of energy and motion because of the harsh texture and shape of the foreground rocks. Fast moving clouds in the sky also add more energy and tension to this scene.

Technical details:

Camera	Nikon d810
Lens	Nikkor 16-35 f/4
Settings	ISO 64, 19 mm, f/1, 241 seconds
Other	Single shot, ND 10-stop filter

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Anshar Photography
http://www.ansharphoto.com