This study presents an encapsulated monopole antenna with a simple, small, and compact design to provide high reliability and security in unmanned surface and underwater floating systems. It is designed and implemented at a frequency of 2.4 GHz with reduced mutual coupling between antenna elements. The proposed design uses two radiating elements with slots in the radiators to provide high isolation between the antenna radiators and suitable encapsulation materials to optimize performance, secure communications, and increase reliability in AUV. The proposed antenna has a compact structure with dimensions of 11.4 × 5.3 mm, which is the smallest size compared to previous antennas. The proposed compact circular antenna with a common radiator operating at dual frequency is different from traditional antennas, the radiator is shared by two antenna elements, which greatly reduces the overall size of the system. A high level of matching is achieved by the dumbbell-shaped radiator and the extension of a stub on the ground, and the common radiator is fed by two perpendicular feed structures. The diversity antenna is implemented on an FR4 epoxy substrate with a dielectric constant of 4.4 and a loss tangent of 0.02. The overall volume is 11.4×5.3×1.6 mm, operating at dual frequency of 2.4 GHz and 5.8 GHz with good return loss of close to 25 dB. The proposed design is investigated in terms of S-parameters, diversity gain, radiation pattern and coverage correlation. Compared to the direct antenna element, a -20 dB improvement in isolation is observed for both simulated and measured results.