i cant speak for dave (DD1), but the thoughts i had were:
a) (that unless your pond substrate is good thick compacted clay), even if you core the dam at a "downstream" location, after pond fills, the downward hydraulic pressure of your pond water could cause reverse flow of water table upstream of dam and drive water beneath or below your dam and out the other side
b) the high water table conditions would not allow proper compacting of core and could create a soft condition at base of your dam....which could be structurally significant.
i'm just a guy on the internet.....your contractor has built other ponds in area, and should know best, but you might ask him about these couple of concerns.
