When I was a lot younger I built quite a few plastic sailing ship kits to sail. The casualty rate was high.
Ballasting such a kit is a real crapshoot. There's just no practical way to figure out how much ballast will be necessary. Every additional piece of styrene, wood, or whatever that's added to the model will add to its weight - and change the center of gravity.
I suppose one way to do it would be to add weight to the exterior of the hull below the centerline of the keel.
Having owned a number of pond sail boats as a kid, its most practical to have a narrow (front to back) fairly deep extra keel.with a weight on the bottom of it. Not prototype but it cant be seen.
The subject of the relationship between scale and water is an interesting one. You can change the scale of your model, but not the water. I have no idea how many modelers have discovered that a scale keel just isn't up to the task of keeping a scale model upright in any sort of breeze. Many modelers add "false keels" to give the water a little more of a fair chance.
The classic Revell America comes with an add-on pond sailing keel, but its thin plastic and my guess is that the model would float nose downward.
When my little model of the frigate Hancock was almost done, out of sheer stupid curiosity I took a deep breath and set it afloat in the bathroom sink. Much to my surprise, it floated upright - but about 1/4" too deep in the water. I haven't floated it again.
Probably a very wise choice.
Watching a little sailing ship ghosting along the surface of a pond or swimming pool is indeed a big pleasure. Just don't expect much in the way of performance - and pick a calm day and a shallow-draft pool. When I was at Ohio State, there was a nice, landscaped pool called Mirror Lake at one end of the campus. My dad and I would sometimes sail models in it - and we usually drew quite a few interested spectators (including golden retrievers and water spaniels). The most spectacular event of that sort, though, came when my Revell Beagle sank, and Dad (a fearsome Professor of Architecture whose students quailed when he approached) went wading after it. Ah, memories....
There were a couple like that in Architecture school. One guy had a six or so foot long pole he stood with like a spear. Whenever he wanted to add emphasis to a point, he'd bang the butt of it really hard on the floor. Another structures Proff. made us stand up and click our heels together when he came in.