If you have a fountain, don't do waterlilies as they like still water. Water hawthorn looks unusual and smells fabulous. Myrophyllum does well with me for a deep marginal but I live in the south. you have to be careful not to let it escape into the wild though as it can be invasive. I grow schizostylus (kaffir lily) as a shallow marginal. Iris Kamferii (SPELLING!!) the japanese iris comes in plenty of colours. Best suggestion is to find a water plant grower near you and find out what does well in your neck of the woods. The other thing thats really easy is the water forget me not