Pirl is right - you don't need a bigger bathtub. I was trying to determine if you hit clay or hardpan and how far down it was.
If your other Hydrangea don't have this problem, it might be localized enough to deal with. Sometimes you can break through the clay/hardpan or break it up and remove it. I had some pretty heavy garbage cans for the first year we lived here.
First, are there ways to minimize the amount of water in this area? Is your house downspout right there? Worse case scenario, you trench and put in a French drain to carry water away from your garden and disperse it to other less sensitive areas (like under the lawn).