Otters can go over hill and dale for some distance. But, you are right, they must have come from some other body of water. Living in Maryland I can't believe your are not close to another body of water, small though it may be. If your pond is wild, that is, you are not worried about gold fish or stocked fish, I think the otters will leave enough fish behind. I see them work ponds about 12 acres or so all the time, leave, and then a week or so later come back for more, and then come back year after year.
Otters can go over hill and dale for some distance.
But, you are right, they must have come from some other body of water. Living in Maryland I can't believe your are not close to another body of water, small though it may be.