Nothing kills things like wet feet (alternating with having their roots frozen into a popsicle) in winter. I don’t know how high the water is and how swampy you get in winter, so hard to say what to do, but the basic guidance is don’t fight the site. Any chance you might prefer some more wet-tolerant plants like winterberry holly (Ilex verticillata), or buttonbush (Cephalanthus occidentalis) or summersweet (Clethra alnifolia) or even the swamp azalea (Rhododendron viscosum)? Obviously there are other choices; just naming a few. The "how high" question, without seeing the site, I guess is best answered like this: High enough so the plants think they’re not living anywhere near a high water table. Meaning not in the native soil, but in what would amount to an artificial berm or raised bed so high that it wasn’t affected by changes in the underlying water table. Hard to imagine that’s going to be very easy or natural-looking to accomplish. That said, depending on the architecture and how much you want to invest (or how handy you are carpentry-wise) you could build large planter boxes along the lines of ones that might be on a roof deck.