It mite have mites (sorry, I had to do that) or be in the shedding process. I sort of see that with Charon (my hissing Hogger) except he holds his breath under the sand (crushed English walnut shells actually) instead of the water dish. He seems to stay under there for hours (maybe he takes a breath every once in a while when I'm not looking). I can tell he's holding his breath because when I pick him up from under the sand he doesn't hiss at me until he has reached the surface, and he needs to breathe to hiss, right?

P.S. Charon is less than half the size of your boa, as you can see in my sig.