Ooh, agree with all of the above posts. MONTY DON has written an article on his gardening regrets in this month's Gardeners's World Magazine and a willow hedge is one of those regrets.
Few shrubs like soaking wet feet but there are many that like moist, well drained soils. Sambucus, rhododendron, witch hazel, Leucothoe, hydrangeas and Cornus like moist conditions and usually acidic sites.