They made the foxhole up in order to introduce the idea that there were people on the shore who could/would shoot. Of course there is no reason for a mine to be detonated by someone on shore. There probably wasn't anyone on shore.
Any idea what the water table is like so close to the delta and so close to the river shore? How deep would one have to dig before the hole begins to fill with water? Is it anything like digging a hole at the beach?
-PJ