Look into racerstar. They make good motors for cheap.
I have this one on my explorer and it has unlimited vertical.
Racerstar BR2830 1300KV 2-4S Brushless Motor For RC Airplane
https://banggood.app.link/WuYWYwkZuR
On 3s with a 9050 prop it's pushes 1130 grams of thrust and only draws about 22amps. Very efficient for the power you get. And with two on your seaduck, that's 2200grams of thrust! Don't think you'd have thrust issues... Lol
I hope to get a FT Legacy for Christmas (I've dropped about 20 hints to the wife, even linked the pre-sale page to her in a text lol) and my plan was to put two of these in it. I'm my opinion it's a very comprable motor to the FT C pack motors by emax at 1/3 the price.
Enjoy the build post pics of your completed duck on the forum please!! I want a seaduck pretty bad but don't have any water to fly it on near by...
Edit: I just saw that you were already looking at this motor. Lol It's a good choice. I'd personally recommend this motor on 3s with 9050 props.