Okay, it's not the camera! I swapped to my back-up for the Channel Islands (pics to follow when I get recovered from 3 days of 5-a-day diving) and still had the spot. Checked my port, and couldn't find anything, so I cleaned it again. Had my little focus flashlight turned off for the whole time, as there was a ton of light. It wasn't that. I'll have to look at my last set of pictures, after I recleaned the port. Otherwise, someone on the boat suggested I might be getting a reflection of my strobe inside the housing. Or maybe I'm getting a spot from the infrared (laser?) focus finder on the camera? The fact that it's always the same size and always in exactly the same place makes me doubt the reflection theory. Hopefully, it's gone since I recleaned the port. Tiny nick in the glass maybe? Blah!