Well a mine placed with killing a sub in mind can be highly effective. mines land or sea are much like a machinegun set up in a sighted zeroed fixed postion. Anyone worth thier salt is going to place them in certain areas to deter action in fact on land you can use the mines to force the attacker into your machineguns. They could set the mines so that they would get set off by a submerged submarine that is very effective as a sub now has to take risks when he is a possiable mine feild he had to stay on the surface and take his luck with planes spotting him if he dove he may hit a mine.