When there is the danger of drowning you should do two things. First you should know that your body itself is light enough to float, and second, make sure you can’t just stand up.
While it’s definitely not funny to the boy, it’s somehow funny for everyone else, as he holds onto that rope above the water which is approximately only a foot deep. Once someone helped him get back up into a vertical position making it suddenly all okay again. Pretty much: “OMG! Help me, I’m gonna drown! I can’t hold on much—oh …”