In the middle of canals theres a bridge with grenades on it, so if you go up the stairs and you keep on going straight, you qill see this little square box with a tree in the middle.
Take cover right by the white edge and press A and B at the same time while going into the little corner of the brown cover and the white edge.
You will get on top of the white edge and just roll into the water.
There's no way to get out but it's fun to do especially if there's little time left and you need a draw.