Delphiniums would not be my choice for your situation. Although they do appreciate moisture, they also require very good drainage (as most other perennials do). Instead of delphiniums, try perennials that don't mind wet feet. Those that can survive wet conditions include cardinal flower (Lobelia cardinalis), Japanese iris (Iris ensata), Siberian iris (Iris sibirica), chameleon plant (Houttuynia cordata 'Chameleon'), common rose mallow (Hibiscus moscheutos), meadowsweet (Filipendula), marsh marigold (Caltha palustris), and quamash (Camassia quamash). To help increase the drainage, you can plant in raised beds or on berms. Soil that has been raised even a few inches above the rest of the ground will offer much better drainage.