I’ve been using a new toy lately, the Lee Filters Big Stopper, which effectively lengthens any standard exposure by a factor of ten stops. So, in the case of these images, 1/30th of a second becomes 30 seconds. You need to use manual focus and exposure, but the results can be very effective and give a ghostly ethereal feel to moving subjects such as water, especially when shot against rock solid stationery elements. I have no axe to grind when mentioning this particular manufacturer. They just make the best (albeit most expensive) filters out there; and they’re British.
These were taken on the South East coast at Seabrook, near Hythe in Kent.