If there's any way to raise the bed a bit to improve the drainage, azaleas would work - it sounds like just the sort of conditions they like, moist, shady and acidic! Same with mahonia and loropetalum. For flowering plants, I'd try columbine, oxalis, sweet violets, caladiums, wandering jew, ferns, turk's cap and gingers.

If there's no way to improve the drainage, and it's going to stay moist all the time, your choices are a little more limited. You might just have to try stuff and see - getting plants at the swaps would be a good start, to see what will grow well. If it's REALLY wet there you might look into bog plants, like pussy willow and arrowhead plants.