Things that go "Splash" in the night

The next nights camp is less inviting, being an river dug out to flow under the main road. I pitch my tent in a field full of cows and am given a strange warning by a passing fisherman complete with mimes of teeth biting and wild gesticulations towards the Agua (water) - the only word I understand in sentences I know I would be better off gleaning more information from.

Anyway deciding that maybe I won´t go for a swim, I settle down for the night. Then comes the first SPLASH of something large in the water. At first I think someone is throwing things off the road bridge above to scare me. I sit freezing in the growing gloom to try and detect movement - nothing. But with an intelligence that is worrying, the splashes only recommence once the tent is zipped up. I reason that nothing has tried to eat the tent so far and install headphones so as not to hear any more. A good ploy as I awake the next morning, alive, and with my MP3 still on. Never did get to see what lurked beneath, but something big and with teeth if the fisherman is to be believed.