See no problem to that just cover the hole in the bottom if there is one and at the end of the season take them out of the pond for clay pots will crack in the cold weather which means any aquatic plant in the pot will be lost if not repotted in a plastic pot and set below the water level. Oh my, that was probably the longest single sentence I have ever written:) Welcome to GOY!

I use clay pots in the pond to give different levels and they cause no problems I also leave them in situ all winter. they don't freeze as the pond hasn't frozen to any great depth for several years.

welcome to GoY from me too.

Yes! I have some in my pond where I didn't build shelves for my prepotted marginals. I also have a floating pot that you grow marginals in and floats about the pond like a little island :)