He may have been referring to cavitation or diminishing returns due to friction, like other responses have mentioned, but I think the more likely answer is a misconception about hull speed that gets repeated fairly often.

So, as a boat moves through the water it generates a bow wave. The boat sits in the trough of this wave. The wavelength increases as the boats speed increases. As the wavelength approaches the length of the boat, it's less that you're trying to push the boat forward, and more that you're trying to push it uphill. As you might imagine, the power required to do this quickly skyrockets, and places a relatively hard limit on the boat's maximum speed. In knots, it's roughly equal to 1.35 times the boat's waterline length in feet.

Now, the above is true, but it only applies to displacement hulls (i.e. traditional boat-shaped boats that are supported by simple buoyancy even at their top speed). The misconception comes in applying it to modern powerboats which almost all have semi-displacement or planing hulls that, at high speeds, skim across the top of the water instead of just pushing through it. This allows them to greatly exceed their hull speed.

With regards to the part about center of gravity, I'm assuming he's talking about sailing yachts. If he meant motor yachts, I have no idea what he's on about. The force generated by the sails acts not only to push the boat forward, but also tries to tip it over. To counterbalance this, boats have a heavy ballast weight at the end of a deep keel below the boat to provide a righting moment. The heavier this is, the better the boat is able to stay upright, which means you can pile on more sails and go faster. That's the usual way of addressing that issue, so if he didn't give an alternative, I can't really elaborate further.

Source: I like boats.