These beautiful slow flowing waves can be seen on the shores of the Nantucket island in Massachusetts in the winter. The frozen waves are actually part-frozen as they still move, though hard and slowly.
Many photographers get obsessed with the phenomenon which is also often called "slurpee waves".
These ice crystals are called frazil. Interestingly, when that ice is formed, it can become a source of drinkable water. The salt that doesn't freeze is left behind. This natural phenomenon can occur when the weather meets certain conditions. These are the freezing air temperature, low water temperature, and motion in the water. The most suitable time comes during really cold days of January or February. However, keep in mind that ferry schedule is not so stable during this time because of weather, so it's better to opt for a plane to get to the island.