What is the layout of the pond like? I’d guess it doesn’t have steps/shelves at different levels to sit pots on. A lot of my pots are propped up on bricks, maybe
Try something like that. If your pond is all one deep level of 4 feet maybe try putting something like an upside down milk crates on the bottom then some bricks on top to make a platform to sit a large pot on top and have it closer to the surface of the water. I have Japanese and siberian irises that have their crown several inches underwater so I would think cattails would be able to handle being submerged deeper even better than irises which I have growing also in “normal” gardens with average water levels but I have never seen a cattail do that.
Can you tell anything of the size of the plant yet or is it just a dormant ball of roots? Such as if you guess if it is some of those cute little pygmy cattails sold specifically for water gardens and say only a foot tall when mature or those big monsters I see all the time in marshes and drainage ditches? I would bet those monsters could take being submerged a couple feet easy.
Hey, if you have two and they were free I would at least test with one to see if you can rig something in your pond so even if it is way too deep and dies at least you know and have one left. To me it would be a major PITA to try to put it in a pot on shore and keep it wet enough, too much of a hassle. I have an arrowhead plant that was supposed to be a marginal plant sitting along the edge on a shelf only a bit below water but it fell into the deep 5 ft section years ago and every year it shoots leaves to the top just fine even though I have a feeling that it is much deeper than suggested.
I would go with a nice large plastic pot, with a lip preferably to help pull slippery, algae covered wet pots out when necessary to divide plants. When I pot them I just use a layer of gravel on the bottom of the pot to keep smaller particles from slipping out, then a layer of plain, non-scented kitty litter (the super cheapie stuff from a dollar store) which turns to clay when wet to keep dirt from slipping out. Then a bit of any old, non-rich soil so there is not a ton of nutrients to maybe cause more algae that I dig up in the garden and put the root ball in this layer. Then a bit more kitty litter and then top off with gravel again to keep everything in the pot and try to not have a bunch of dirty looking water as soon as you put the pot in. Then I usually pre-wet it with a hose to get the dust out and settle the soil otherwise sometimes stuff starts floating out more if I put the pot in the pond immediately with all the soil components dry.