By all means, keep an eye out for mites, as new ones could have hatched from eggs that could have hitched a ride. But unless you find some, I wouldn't worry too much about your snake being in the water dish. He's still really new to you, and still getting used to a new environment with new hides where everything is different. That takes awhile; mine won't sleep in a new hide until it has been in there for weeks and he's good and sure it won't bite. So he could just be in the water dish because it feels snug and safe and he's not sure about his hides yet. Or it's possible that your temperatures are actually higher than you think they are, and he's trying to cool off. Or that he's going to go into shed soon. Or that he just likes it there for the time being for no reason you'll ever know because he's unlikely to explain it to you.
Soaking in the water dish isn't in itself a bad thing, especially if your snake is new and still learning the ropes. Getting shipped to a new home is stressful on a baby snake and it doesn't just stop being stressful immediately because your husbandry numbers are good.
Do keep an eye out for mites, but probably the best way to make things easier on your snake is to leave him alone and let him settle down.
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