I wouldn't put the large water area into the tank at all. For one, it will get soiled fast, as dragons do like to poop in water. Also, crickets will drown in it, loose substrate will get into, food etc. Also, there is the risk your dragon may drown, as 8" is deep enough that the dragon can't touch bottom and if it has any trouble getting out of the water, he can easily drown. Also, the humidity can cause respiratory infections or excesses mold growth.
You will be better off just giving him a chance to swim once a week or so. Personally, though the exercise is good, I really don't see any need to give regular baths to bearded dragons unless they are dehydrated, constipated or dirty.
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PHLdyPayne