The Speedboat and SDV should have fully stabilized turrets (stabilizedInAxes = 4 in config, see https://community.bistudio.com/wiki/Turret_Config_Reference#stabilizedInAxes_=_3) however ingame they dont display any kind of stabilization whatsoever.
Whenever the boat/sdv turns or banks over the waves the turret will turn and bank along with the vehicle, if you compare it to an armored vehicle like the marshall for example the turret will remain pointed to the same traverse and elevation regardless of where the vehicle is turning or banking.