Since he's 23 and no longer a teenager, it is pretty rude to treat him as a child, you can only politely say to him what you'd say to any other adult houseguest.
It seems very inhospitable to say, "Listen, you need to leave the shower curtain tucked in like this or water gets all over the floor. If you don't and it floods - YOU clean it up and YOU pay any bills for repair if it damages anything." I certainly wouldn't say that, complete with the caps to a houseguest and would be offended if someone said it to me. On the other hand, a reminder that the lip of the shower is very shallow, so it is important to make sure the curtain stays inside to avoid flooding the floor is reasonable.
If your shower curtain is hard to use or unusually prone to blowing out of the shower, then I think it is up to you to provide one that words, whether that means Velcro, weights, or anything else.
If you really don't have hot water, a warning is polite, on the other hand, telling an adult how long a shower they are allowed to take seems rather rude.