The swans could be either Mute, Tundra or Trumpeter. If there is no water in the area, they could starve, drown if they fall into too small an area and cannot get out, become dehydrated, freeze to the ice or be killed by a predator that can access the birds from walking on the ice. Now, having said all of this, swans and other wildlife are not going to do anything stupid that would jeopardize themselves or the flock. So, you may not be able to see water, but there may be a small area which would allow the birds to stay in the area. The birds may also be traveling to and from the open bay, not in so many numbers that you would notice.

Additionally, if the birds were waiting for a break in the weather or were blown off course, they may be getting their energy back and will fly off in the next few days. The Regal Swan