Pro tip:

Stop looking for affirmation on the internet

The kid doesn't meet your standards. But then, you didn't hire him. Get used to it. Someone wants/needs him onboard.

when you work small crew vs big corp you are going to get a lot closer to people you work with especially when you are weeks on

try and help him , just don't get too invested in him actually taking your advice.

a lot of the advice you got here was probably directed a lot more at traditional work environments where people are there for 8-10-12 hours and gone and there will be a new guy to take the last ones place in a few months any way.

Exactly! I think it has to do with metabolism. If you have a high metabolism, energy drinks may not be for you. If you don't, you can probably drink several and go right to sleep half an hour later.

Let's see, he's 19 and you're what? 25? And you're his boss. You're some kind of middle management and he's a problem to you?
Rather than bring up and expose his business to the brain trust of a large outdoor sports forum wouldn't it be easier and more ethical for you to go to your immediate supervisor'(s) and tell them you are having issues with this young man? Thought I'd heard you mention you were in some branch of the military. Ever heard of 'Chain of Command'?

that might actually be , becoming lethargic from overly high blood sugar , when you can't produce the insulin to process the sugar you get very tired.

so the caffeine doesn't seem to effect you might be that you are shutting down your pancreas and you don't notice it

You got a good heart worrying about the kid. But we all have free agency to make our own bad choices. If you've already explained how his habits are destroying his health and possibly endangering coworkers several times, and in different ways, continuing to do it is just nagging that will not help either of you. If he is creating hazards tell the Cap and let him deal with it.

The recent energy drink craze will be the cause of type two diabetes in many of this generation. You can't keep slamming your system with those high doses of sugar without overwhelming your pancreas leading to insulin resistance in all your tissues and diabetes. If you're drinking any sugary drinks (including sodas), at the very least, eat a high protein snack (nuts, jerky, cheese stick etc.) with them to lower the glycemic index. Am I nagging ?

Yip, like I said..It sounds a lot more like control, than concern

Sounds a lot like the mandatory helmet laws crew in reverse.

If that happened on duty, the captain either knows about it, or needs to have been told...If nothing was done you have a lot more problems than just a deck hand. That should have been addressed by superior officers immediately.