How long has the pond been running? Cement can leach out chemicals. If it’s well established, you’re good. If it has good sun, you might research various water lillies, they bloom, are beautiful, but require a pot, substrate ( pure cheap clay cat litter works) , and you’ll have to pull them every few years, and separate the rhizomes into separate containers or give part away, otherwise they can grow to a point that might escape the pot and grow where you don’t want them. If this pond has any additional substrate, you can pick just about any plant that grows submerged in your conditions. I’m several states away, so don’t know your weather, water ph, etc. Just about any floating plants should work, again take weather, ph, into account. A good idea would be to look if there are any pond stores near you. Ask what does well in that area.