Floating ocean seismograph (FOS) is a vertical underwater vehicle used to detect ocean earthquakes by observing P waves at teleseismic distances in the oceans. With the requirements of rising to the surface and transmitting data to the satellite in real time and diving to the desired depth and recording signals, the depth control of FOS needs to be zero overshoot and accurate with fast response. So far, it remains challenging to implement such depth control due to the variation of buoyancy caused by the seawater density varying with the depth. The deeper the water is, the greater the impacts on buoyancy are. To tackle it, a fuzzy sliding mode controller considering the influence of seawater density change is proposed and simulated in MATLAB/SIMULINK based on the variable buoyancy system and state space function of FOS. Compared with proportional-integral-derivative (PID) controller, fuzzy PID controller and sliding mode controller, the simulation results indicate that the proposed controller shows its superiority regardless of the disturbing force. Its advantages include smaller steady-state error, faster response time, smaller system chatter, and well robustness. This proves that the designed fuzzy sliding mode controller is able to meet the working requirements and thus, lays a foundation for FOS application.