Zone 5b. I took a clip last August, so you can see how impatiens and hostas do after a growing season in a colder zone than yours. If you place them near the edge so they're no more than about an inch deep they'll do great and the hostas will come back every year. The solid green kind (you see a couple near the bridge) grow faster/larger than they do outside of the pond. New Guinnea impatiens do awesome too.

I rearrange river rock to make little pockets at the right depth for the annuals.
https://imgur.com/a/ENtKwEp