You have got to give the competition credit for their endless attempts to get around the expense of building their boats with combination keel/centerboards. First they went to swing keels. Then wing keels. In between some few tried dagger boards. Now it's long centerboards with interior water ballast. Everyone of these approaches had merit. They also had strong drawbacks and thus the need to keep trying again every few years. Unfortunately, if you own one of these brands with last year's keel/ballast attempt, your resale value plummets when your builder brings out this year's system with the subtle suggestion that what they put in YOUR boat was not the way to go.
The Rhodes 22 came out with a combination keel centerboard on boat number one in 1970 and continues with this proven system in its latest models. Rhodes resale value is the highest in the industry.
Ballasting is accomplished in several forms: Live. Internal. External. "Live" ballast is all the small boat has. Where you sit does the job. All big boats use external ballast. In between size boats are best served by a combination. In its exclusive design, the Rhodes provides a flared hull with very comfortable built-in hiking seats, cabin back grab rails, externally mounted stern rail and molded-in heel support. Shape and hardware are coordinated for live ballast as a viable option to uses when you wish, as an adjunct to your solid, EXTERNAL ballast.
External ballasting is more efficient than INTERNAL ballasting. Water ballast, of course, is internal ballast. To get the same effect more weight must be used. More weight means more of the boat under water and more wetted surface or drag. It also means less room inside the boat for you and gear and equipment features. Water ballasted boats you are considering do not have keels so they must use centerboards. Very long centerboards. Five to six foot long centerboards. This means that, while water ballast advertising says you can now float in less than two feet of water, it does not follow through by reminding you that you need very deep water to sail with the board down.
There are advantages. Water as ballast is less costly. The trailering load on the highway is less. But pulling the boat out of the water is a greater strain than an equivalent externally ballasted boat since the water does not empty out of the internal tank until the boat is out of the water !
Actually, expensively mounted water ballast systems have merit. Water tanks should be on each side of the hull so one tank can be ballasted while the opposite tank is pumped empty, depending on the tack. Low cost systems are in the boat's center and must be fully loaded or completely empty to avoid a shifting ballast that would actually make the boat more tender than no ballast at all !
We facetiously say that if you still want water ballast just drill a hole in the Rhodes bilge(the Rhodes is unsinkable). But, in a way, the Rhodes does combine water ballast along with external and live ballast. A 15 gallon galley water tank (compared to none or 5 gallons on other boats) and a waste holding tank (not found on other similar boats) do add liquid ballast in port and starboard controllable tanks !
So, if you are debating "water ballast - centerboard" boats vs. "combination keel/centerboarders", read Practical Sailor and all the other sailing publications that have written on this debate.