This paper presents an evolutionary topology optimization method for applications in design of completely submerged buoyant devices with design-dependent fluid pressure loading. This type of structures aid rig installations and pipeline transportation in all water depths in offshore structural engineering. The proposed optimization method seeks the buoy design that presents higher stiffness, less material and a prescribed buoyancy effect. A hydrostatic fluid is used to simulate the underwater pressure and the polymer buoyancy module is considered linearly elastic. Both domains are solved with the finite element method. From the initial design domain, solid elements with low strain energy are iteratively removed until a certain prescribed volume fraction. The studied case consists in a buoy for supporting subsea oil pipelines, in which the inner diameter is constant and the outer shape and interior holes are defined by the optimization algorithm. A new buoyancy constraint is introduced in order to guarantee a satisfactory buoyancy effect.