Could be mites, but if not.. What are the temps in the water bowl? Is your ambient humidity low?
It could be that the water bowl is a hide place (in the boa's mind) at the temps she is looking for. Boas do use low temps too, and by "low" I mean lower than basking temps of course, not low like colubrid snake temps low.
And of course, boas love to hide in tight places.
So, if a boa finds a tight hide at the temps it wants, there is where he/she will be. You can try a tight hide near the water bowl and see if the snake uses it.
Generally, a boa can hydrate itself very well by soaking for no more than 10-20 minutes. So hydration is likely not the reason the boa is sitting in the water for that long (unless the boa has mites or some other thing going on where water helps.) My guess anyway from the other end of the WAN.
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Mark