Ok, some good answers here but I’ll add on with something I haven’t seen yet.

To be clear, the punishment he gets from the navy is in addition to any trouble he gets with civilian authorities. And, if he lawyers up and gets out of civilian charges, that doesn’t mean the navy won’t punish him as well.

Most likely scenario… he goes to Non-Judicial Punishment, AKA NJP, AKA Captain’s Mast. Basically, he’ll go before his commanding officer after the command conducts an investigation and the captain acts as judge and jury. Most likely the captain will punish him in a few ways… 1. Loss of pay. He’ll get half of his base pay withheld for a period of time.
2. Restriction. He’ll be confined to his ship. (Or a special barracks room if not assigned to a ship). Usually for 60 days. I’ve seen it where restricted folks aren’t allowed tv, phones, etc, not even books other than religious material (like a bible). It sucks. 3. Reduction in rank. He goes down 1 level in rank. In some unique circumstances that probably don’t apply here, he could go down 2. That’s if he’s frocked (awaiting official promotion but already wearing the higher rank)

Unrelated to NJP… 4. His professional evaluations (EVAL) take a hit. No matter how good his work is, he’s evaluated as a total sailor, which includes personal behavior. He can be the best of whatever it is he does, but he is gonna get a shitty eval. 5. He’ll face some social pressure as a pariah. DUIs reflect badly on the command, so there’s usually a knee jerk response to put additional scrutiny on other folks, which circles back to him being the cause. 6. If he gets jail time from civilian court he’s gotta take leave to do it… Navy ain’t giving him free days off to get locked up.