There are problems associated with this sort of conduct.  The reported ones are underage drinking and littering.  Sounds also like child endangerment as to the minors.  

OK.  Here's a solution.  Make all participants either criminally liable for the specific acts, if caught, or conspiracy to commit those acts -if floating .  Make the only alternative for "avoidance" of charges for any of those things transpiring NOT TO PARTICIPATE.  Simple.  

Sounds harsh, but this has to stop.  It's idiotic because it requires the intervention of law enforcement and life saving services to avoid death due to drowning, caused by use of alcohol at the very least.  There's also the trash problem.  

Just make people criminally responsibleas "conspiracy", round em up as they exit the water, and charge all if there's a single incident of litter, alcohol abuse, underage drinking, or child endangerment. 

They glory in the group entitlement to do it.  They should take group responsibility for the problems.