I'm replying to you because at least I know you come from a place of emotional stability and reason.
Any party in this absolutely should seek legal counsel, that shouldn't even be in question.
Is it possible the young lady drowned? You bet. Is it possible that the boy tried to save her and had to basically give up due to fatigue or sheer exhaustion? Yes. That does not, however, trump the fact that the boy gave multiple conflicting accounts to police and made the mind bending decision to walk past the front desk in light of reporting the events and going straight to his room and sleep it off. There is no investigator whether PN DICRIM, FBI, Interpol or otherwise private, that would not be weighing this against the boy's credibility. Furthermore, not only did he not report this, he only gave the first account when questioned by police later in the early evening that same day.
Sure, it's possible the young lady was intoxicated and drowned, but from an investigation standpoint, the boy's actions and stories leave significant doubt, enough so to keep the investigation going and him detained.