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.
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.
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.
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....
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.