I think most new keepers cause much more stress than they need to. Try to relax knowing you are trying your best and we'll get you squared away. The snake is probably just being a snake and I sometimes think they try to make new keepers go crazy.... Like they know.
I doubt the snake was going to drown itself. My BRB has eaten an entire rat under water.
As for mites. If it's eating I would say that's not it. You can also see the little buggers so without seeing any little pepper like spots in the cage or water again I don't think that's it.
Now it could be you changing things in the cage. Maybe the hides are not giving a nice secure feeling. A trick I've used for years is adding moss to my hides. Especially on hides that are larger than the snakes needs. Both of my Kings are little right now with hides that would be considered huge for them. I put a large clump of moss in them and it makes it like a cave system. They can borrow into it, climb on top, go under, you name it. It just gives a much more secure feeling and it's super easy to do. Plus during a shed you can mist it and create humid hides. Even with my Ball I keep clumps of moss around the opening of one of ours hide to make it a little more closed off. I'm doing that with my Madagascar Hognose too. I also have two hanging hides that are too big for the snakes they serve. Filled them with moss and now they're just right.
But then it could be that it needs a bit more food. Maybe try feeding a bit more often or a little larger food item. I have snakes that act like they are hungry the day after I feed them. Like my bulldog you would think I starve them.