There are a few filters (that I know of ) that are useful for sunrise/sunset shots ND Filter
work like sunglass tt blocks the light enter into the lens. It is useful to stop down the shutter speed like 1sec or slower to capture smooth & silky effect of the sea
Graduated ND Filter
balance the exposure between the bright sky and dark ground. It is the square/rectangle filter type with HOYA or LEE filter holder allow you to adjust the horizon level easily.
Cut down the unneccessary reflection on the sea.