This paper considers the control of one unmanned semi-submersible vehicle (USV), named BQ-01, where the submersible depth cannot exceed a few meters in the presence of environmental disturbances. Consequently, the depth control performance measures and the rejection of disturbances, such as those due to sea waves, must be within pre-determined ranges to ensure maximum use of such vehicles. This paper develops a control scheme that combines the state feedback control and rule-based supervision for depth control, where the entries in the feedback control matrix are adjusted by rules formulated from the state errors in the USV diving plane. Surge speed also plays an important role in adjusting control parameters because of its coupled relationship with the state depth of the diving plane. The results from sea trials are given and confirm the potential of the design.