This happened a couple years ago back when I was in college and is to this day one of my favorite sexual experiences. The 3 below sections just explain the situation and unexpectedness of it all if you want you can skip those to get to the foreplay.
Every year (until recently) Ohio State University has a tradition called the mirror lake jump. Basically a bunch of students jump and party in their on campus lake called Mirror Lake before their biggest sporting event of the year, their rivalry OSU vs Michigan football game.
I was visiting (as I didn’t go to OSU) and staying with 5 guy friends that lived off campus to partake in all the fun that weekend. We pregamed, got drunk, and then around 9 PM after the sun had gone down made our mile and a half trek to the lake. By time we got there the lake was already bustling with activity and a quarter full. In my experience it only fills up about halfway as the other end is quite deep and drunk college kids tend to do better when they can stand as opposed to trending water.