If a 12 years old visitor drown in the river of near my hotel although I created the warning sign, am I responsible?

He might argue that you allowed an attractive nuisance [the river] to exist and should have fenced it off. If your sign was obvious enough, and especially if he saw it, then you might not have any liability. You do not need to post a lifeguard for a place that no one is supposed to swim in, unless perhaps you know of other kids who tried to swim there. I assume you have liability insurance, but if you tell them before any claim is made, they might up your rates or drop you. Take pictures of the sign and anything else indicating people are not supposed to go in, off the river bank to show it would not be safe to try to go in, the river its self to show a 12 year old would realize it was unsafe, get statements from anyone who saw what happened, be nice to his parents so they do not get angry at you, put up a fence along your property fronting the river and add more signs saying it is dangerous and give each future guest a written notification not to use the river and make sure their children don't.