You can buy a handful of cut watercress in the fresh herb/veggie section of a large grocery store. Stick these in water and they will root. Put the rooted plant without soil in the pond with a rock on it to hold it in place or stick it in a pot. This watercress will grow quickly when the weather warms up and is beautiful. It is a low growing vine and will cover the surface of the pond, hide pots where you stick it and so forth.

It is a perennial here in Barrie, just north of you. It comes back every year for me and flowers beautifully after the first year. You can also cut it continually for salad. My favourite water plant and cheap!

Cannas lilies are also a good pond plant but need have their base in a pot of soil. They love the pond!

Look for "edoe" in the grocery store. Its a brown root kind of thing and smallish. It a type of taro but cheap. need to be potted in soil in the pond. Mine rooted and grew quickly this spring from the store. You will have to bring the roots in for the winter.

Mint is another very cheap pond plant, but invasive.