130 operational DD's?
well, this means shooting a DD in campaign would really be worth something.
About that comparison to u-boats:
US subs couldn't co as deep as a U-boat, this is primarily caused by their construction:
In technics, you calculate the thickness of the material e.g. the pressure hull by dividing the maximum pressure a material can handle by a safety value. For example: if you want 2 times the safety against destruction, you play as if the material you are using is only half as durable as it is and therefore use double the material.
.....this safety value was 1,8 for US subs and 2,5 for the u-boats, so a u-boat could take more pressure and dive deeper.
However, US-Subs weren't required to dive deeper, because this was mostly impossible in their areas of operation. And well, if you have seen what shallow water can do to you in GWX, you will notice that this can make SH4 really dangerous.