Wow. Try this.
Take an old salt bucket or anything like that, fill halfway with water. Put the sock in it and ring it out a few times in the water. Do that right side and inside out a few time. Let water sit a few minutes and see if you have sand at the bottom of the bucket. If the sock was full of sand dust, that is a pain to get out.
If it is then you will need to wash the sock again, and you can beat the sock on a sidewalk or driveway while washing it. Ringing it out both inside and right side out during this process.