Considering your location, you should have a much easier time getting plants to grow for you! Everything I plant has to be able to withstand a hard freeze, so it limits the flower choices I have. All of my plants are directly in the rocks -- there are no pots anywhere.
There are a lot of plants that grow well in marsh-like conditions. Anything you see that grows near the water's edge can be planted in or beside your pond. Something like an iris likes to get its roots down into the water. The trick is to see what conditions the plant grows in naturally. Some will grow in shallow water, while others cannot have their crown (the ball where the roots come together at the base of the plant) submerged. Put plants along the edge of the water if they need to be up higher, or you can use a small pile of rocks to hold them in place. Once they get going, they'll adjust themselves to where they want to be, but you have to keep them alive long enough to do that.
The thing about plants is, once they get their roots in the water, they will start sucking up all those free-floating nutrients (what we call fish-poo) and thrive on them. Just start with some native freshwater plants that you find in the area. Last year I went on a search for snake-grass. I knew it grew around here, so I went driving for an afternoon. I ended up finding the stuff growing in ditches where they got water runoff from the fields. I dug up a couple buckets full and brought them home. They only had about a month to get established before the cold weather hit, but this Spring I expect to see them popping up all around the pond. These are one of the plants that don't like their crown wet, so I tucked them all around the edge of the pond, some propped up in the rocks in the water, and others tucked just on the outside of the liner.
Another thing to consider is methods to change your environment. I practically live in the desert. We have to water our yards to let the grass grow, so you can imagine how hard it is to grow something that needs a lot of water. However in my pictures you'll notice I have a lot of strawberries growing! While trying out different plants, I discovered one of the groundcovers holds the water close to the ground. Any time you reach into the plant, it is always wet underneath. So I grow these and the strawberries together, and they both thrive. Normally cala lilies cannot survive here because of the cold (although if I lived another 200 miles further South they would grow just fine)... So I'm testing an idea to see if I can get them to grow by my pond. Considering we've had an extremely cold Winter this year, if any of the bulbs survived I will consider it a huge success. (It will probably be another couple months before it warms up enough for them to start growing.)
Anyway, the point of all this is to work with what you can find. If you can put the plants into a similar position to where they grow naturally, then the nutrients from your pond will make them thrive, and in return the plants will help filter your water and keep your fish healthy (the fish love nibbling on roots!) If you have troubles, report back and everyone here will be happy to try and help.
