UnderWaterTunnel
Key is that the bottom of the water is 1 meter above the path. Make a 4m path , 6 meter deep in the ground (three steps) and place a 1 meter object. Next, remove the path en digg a hole just as deep as the object. Now, fill the hole with water and recreate the path. Presto! Thanks to PaulsLey for the tunneling tutorial