When I had 3 pullets in with my ducks waiting on a couple of hens to sell to move them over to the other side, I set up a chicken water platform out of the ducks reach. The chickens could hop on it and get a drink, ducks kept their mess out of it. Course I had to place the chicken on it and show them.
Ducks are way too messy to live with chickens full time. You need to remove them from the chickens, into their own house or a temporary cage. Then they'll need an adjustment period. Ducks are finicky, like routine, and hate change.
If you can't swing it or gain their cooperation at all, selling them wouldn't be a bad thing. They do require extra care and time. It's either that, or the cost of getting them set up and several weeks spent fighting with them over the fact that they are not chickens. We had to spend another $200 on a coop alteration and run addition. But I REALLY wanted ducks. Then got some given to me, which gave me too many, and I gave some away to a good home.
It will take them some time to figure out the glorious benefits of a large lake. Ducks look like they'd have some intelligence in those beady little eyes, but it isn't always the case.
Just mastering a ramp into a small pool can be a challenge.
You'll need to make friends with them with food, so you can catch them or herd them into their house at night. A chore, with how suspicious they are. Or they need locked up into their own house, with a muddy pool needing cleaned every 3-4 days, living in oblivion on what they're missing out on with the big pond.
Ever since mine got their baby pool, they don't even care about free ranging in the grass anymore. They prefer the baby pool in their mud pit. As I do garden work, I've been building up the inside of the run with dirt, to make a drainage slope, and then putting the damp bedding from the coop into the run on top of the dirt. At least I can walk in there now without having to wear waders.