Your probably right that it's a retained eyecap, but it looks to me as if the eye and the surrounding scales are bulging. There are probably several retained eyecaps under this one. That, or some matter is causing the spectacle to push outward. Do you have a close-up picture?
Either way, I suggest that you take your snake to a vet right away. Have the vet instruct you on how to remove the cap, if applicable. If there are no vets in your area, follow the instructions that you've already heard.
I personally suggest putting the snake in a plastic tub with damp paper towels rather than soaking it in a few inches of water. Its safer, and the snake is better encouraged to moisten the spectacle.
Best of luck,
Pinstripe
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The snake keepers motto: There's always, always room for one more.