The downside of water ballast is it's relatively low density as compared to something like lead. A cubic inch of water weighs .036lbs, an inch of lead .4lbs. You might do some measuring to see if there is enough space in your bilge for water, it is used successfully on some boats but they were designed around that idea.

I went to school in Boston, I remember it as a great city and enjoying some limited sailing on the Charles.