Hard to hit target. Masked by water. No backsplash near it for rockets to hit and hurt it. Random shots can get it. In the cave, right by that Danger sign, in that dark area next to the rocks.
Someone can make it by running by there.
Grens can easily be bounced back there. As you make a left and there's that narrow wood plank leading up to the grate entrance, it is possible to build a sentry gun along the narrow ledge facing the grate and plank. It almost always knocks the person off and kills him. You don't get the kill for knocking him off. It usually takes quite a long time to find the right spot to build.