I don't go to DC until 2 weeks from today, but I had read previously that you can borrow prescription goggles (they must have many pairs in different strengths). Perhaps someone who has done this can confirm for you.
DC does have prescription goggle that you can borrow. You go to the first gift shop and pick them up. Both my kids used them. I took copies of their prescription, but you don't need it. The lady that works there can look through your glasses and she knows just what you need. My kids were thrilled to be able to see underwater.