Answer: Based on your description of the location, it sounds like a challenging area to establish plants. It is often difficult to establish a ground cover under pines anyway, and the added factors of deep shade and flooding will only compound the problem. My first suggestion would be to place some large containers of shade-tolerant flowering annuals under the trees.
If you still want to try to establish a ground cover, I'd experiment with several different plants and see which one does well before investing in a large number of plants. Lily-of-the-valley may grow if the soil stays moist all summer. Another option is Vinca minor (periwinkle), which is evergreen and has blue flowers. Neither of these plants does well with long term flooding, however. Another possibility is English ivy (Hedera helix). Although this last does not bloom, it is the more tolerant of diverse conditions and might do the best in that location. There are many different varieties of English Ivy available, some with variegated leaves that could brighten up the area.
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