I think it very good that you are looking for a boat 19-22'' for the transition from your Sunfish to your first keelboat. Getting too much boat too soon only flattens the learning curve. It will be quite a different experience sipping sodas with your friends with a dry butt.
Having determined this size range, you will not likely have to concern yourself with water ballast. Hunter has a 24; Catalina has a 25; and you''ve just read about the McGreggor 26. Notice the range? <b>The whole point of water ballast is to make previously untrailerable boats trailerable by adding and jettsoning part of the ballast at the ramp, keeping it off the trailer and off the street. The performance profile is secondary to (a function of) trailering practicality.</b>
(This puts JeffH''s discussion of performance in this context: a water-ballasted boat''s performance is the <em>result</em> of making trailering the priority. If a water-ballasted boat has less draft, it is simply an effect of that goal: trimming several hundred pounds of curb weight).
Since you are not really considering boats in this transitional range, you can avoid the question altogether. There are many great small keelboats in your local market that will provide challenge and enjoyment for several seasons. Most of them have shallow draft and swing-keels, which will simply bump over a sandbar. If you do get stuck, a few cranks on the keel pendant will give you a draft of inches, and you simply float off.
As soon as you are ready for something bigger, you can jump right past those water-ballasted models to something with enough displacement that it lives its life in the water or the yard. (Unless you simply must have a larger, trailerable boat: then these boats become your very narrow range of options).
Good Luck to You