That looks to be little pustules caused by fungus/bacteria from excessive moisture under the scales. In other words,.....stop misting the animal, and put a smaller water bowl in the cage for a while so it can't soak. Then submerge the snake(except for the nose of course) in a shallow solution of Hydrogen Peroxide and Betadine for a few minutes, you'll have to force it down and keep it there with both hands. Then dab some antibiotic/antifungal ointment on all the pustules after the snake has thoroughly dried from the above mentioned Peroxide submersion.
It's not a great looking closeup photo, but that's certainly what it looks like to me.
Decades ago when I had some watersnakes, they would get them as well. You'd think that a water snake would be somewhat resilient to water problems,....but in natural bodies of water, like lakes, creeks or ponds, etc..., there is a tannic acid content in the water that helps prevent this(the reason it's brown)......that, and the fact that the snake can bask itself in the sun to completely dry off periodically.
try to pop, or poke one of the more noticable ones with a small pin, and if it leaks pus,...bingo! you'll know it's pustules.
anyway, do like I mentioned once a day for a week or more, and see how it goes, then you can go from there.
However, if those do NOT start to get noticeably better after a week or so, or you don't think you want to stay on top of it for the duration, I would STRONGLY recommend that you see a good qualified reptile vet. If these are left untreated, or get worse and continue to grow in numbers, they can eventually kill the snake. I'm not trying to scare you, or downplay this thing, But you really MUST make certain it gets better, and not worse, that's all.
If you do as I mentioned, it should work like a champ!..
best regards, ~Doug
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"Better to be silent and thought a fool, than to open mouth and remove any doubt!"