They only like the fresh tang whether it's eaten at low tide or it's thrown ashore after gales. The saltier the better.
Seaweed is just one component in a sheep's diet. If they are allowed to range over large areas rather than fields they will judiciously graze a great variety of vegetation. Finding different plants more suitable at different times of the year.
Heather at first light, sedges and herbs on the paths downhill to find tang at the ebb.