For some time now, I am used to just rinsing my McGizmos, and Surefire's, off under running water in the kitchen sink when they get dusty, or lint on them. I think this light could be rinsed off as well, and you could take it swimming, however, I would want the O-ring installed properly, simply as a precaution, and matter of quality control principle.
I will be getting one from B & H, tell them of the O-ring issue and they will alert the picker and packer via computer. I have done this on lighting Eq. in the past that a great company had some minor QC problems on power supplies, and they did it for me. Now, on a small purchase, I don't know, but you can ask and see what they say. I hate to have to send anything back.
No matter B & H is a wonderful company and if you have never been there, make it your first stop when you go to New York. You won't believe it until you see it!