usv_gazebo_plugin does not apply base_link (buoyancy issue)

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I made a very simple rectangular model as a test model. Then, after writing the urdf and gazebo xacro files, I ran the code, and the position_z value gradually decreased as follows. As a result of analyzing the wamv code of the existing vrx for the application of the plugin, it is estimated that the plugin does not run even though the same bodyname for applying buoyancy is applied. Can you tell me what could be the reason for this?

I use this plug-in(https://github.com/osrf/vrx/blob/gazebo_classic/wamv_gazebo/urdf/dynamics/wamv_gazebo_dynamics_plugin.xacro)