I think this was to show that normal convention was torn apart (like clothes) after the tsunami. THe victims were going to see weird & unsettling things in the aftermath.
I wouldnt be surprised if that really happened either. I could perfectly see something like that happening after a major disaster. To me it seemed quite natural, although the boy seemed a bit taken aback by it...
I imagine with the amount of water plus sharp objects, I suppose something like swimming trunks would likely just get caught and rip off.