What are you using as a heat source?
What are you using to measure the heat?
What are you controlling the temp with if you don't have a thermostat yet?
If you are using a uth your heat souce is defiantly gonna be hotter then 90 and also too hot for the snake. So much so it could lead to burns if you don't put it on a thermostat or rheostat of some sort.
A very common mistake by beginners is using the wrong type of thermometer to measure temps which is why I ask what you are using.
Keep an eye out for mites but if he is soaking off and on for a couple hours at a time like you make it sound instead of just a pretty much constant thing I would think it is something other then mites. I would remove the water fall like you asked about and just put in a regular water ball. He could be trying to use it as a hide like you think if it seems to him like it could be one. Just make sure you replace it with a water bowl big enough for him to soak so if it is mites he would keep soaking and you can still notice it. The dish is the best place to look like mentioned before.
Even one mite is a problem. But if you have only a few mites you wouldn't even notice until their numbers grew. If you have a big mite problem you would see signs in your snake but generally won't see the motes themselves in your tank(or in your snake) unless they are dead in the water. If you can easily see them crawling around the tank or on your snake that means you have a crazy big mite problem.
Take out you substrate if you are using any and replace it with paper towel. Then if your snake soaks it would be easier to find mites in the dish because there wouldn't be bits of substrate in the water to mistake them with. Try to use a while water bowl if possible to make it easier to see mites in it if you actually have any.
You can try to do other suggestions people have said as well in case it isn't mites but you should keep checking for mites until you are certain there are none.