I am not sure if it is as simple as an Article 15.
These men are taught very well NOT TO take pictures on the sub, and he did it anyway.
I know he turned himself in. Which is a mitigating factor.
Not that I would have a problem with it if they did. An Article 15, and tossed off the sub, and put on guard duty in Alaska, but something serious... you absolutely need to send a string message.
Some serious non-judicial punishment would have been acceptable. Your suggestions are good ones. The point must be made not to take lightly any level of classification.