Try some celeriac - they love water and taste good too. I gave up growing them when my water bill almost tripled one year when we'd had very little rain and my water butts had emptied but I had to keep the celeriac going.
I grow my three rhubarbs in quite a hot sunny spot without ever watering and they do really well. In fact, had some for pud today. It gets a little well rotted manure each year which probably helps to retain some moisture. I would imagine that a boggy site would rot the crowns - but I could be wrong.
For flowers, I would plant arum lily - Zantedeschia
Don't be put off by people who say they are for funerals. That's just rubbish. I grow them in a boggy site and they are absolutely beautiful. You can get other colours besides white but I much prefer the white ones. A small plant will quickly multiply and in about 3 years you can lift and divide them easily with a spade and turn your boggy site into a beautiful area. Perhaps some ferns too with some day lillies, astilbies and hostas (watch the slugs). Not forgetting the gorgeous ligularia - one of my favourites for leaf colour. They would all love the bog.