Cinnamon Fern, Ostrich Fern and Royal Fern all like lots of water. Some shrubs might like this area, too: the ornamental, variegated Elderberries, Clethra, deciduous hollies (Ilex verticillata) and some of our native, deciduous Azaleas.

Not sure if hydrangeas are hardy in your area, but check on those. They like lots of water and part shade conditions. Have you considered adding soil to the area? You didn't say how big it was, but I had the same problem in an area about 10ft by 10ft. I added 3 cubic yards of soil and that took care of the problem.

We have the same conditions and have had success with vibernum bushes for larger scale plants. Hostas don't seem to mind the water along with astilbe and ferns. Serviceberry trees are a nice understory tree which do well in your conditions. Both the vibernum and serviceberry have nice white spring flowers. The serviceberry has lovely orang/red fall color and edible berries after the flowers.