They went to go visit family memebers and decided to take a dip in a irrigation canal that was under construction...AFter a while one of the fathers noticed they were missing and found their clothing and bicycles near the canal and they called help and they were found drowned.. Happens all the time
The weird thing is, as a 'former boy' myself, that is EXACTLY the kind of thing my friends and I would have done if given the chance. In fact, I specifically remember climbing into a tunnel of some sort when I was around that age although it was not connected to a drain.