The club barge in the PMBC has two tanks, one fore, one aft, water enters the hull via inlets in the base of the hull, expelling air through a pipe with a valve on it. put model in the water with valve open, allow tanks to fill then close valve. with no way out for the air, the ship can't get any deeper. tp remove model from water, open valve and lift out, allowing water to escape and air to come in through the outlrt.
however on a model like the Happy Hunter or a Neptun, or for that matter the slipway 1/72 puffer, just make the ballast out of batteries and gear and trim with lead