In this letter, a high aperture efficiency (AP), right hand circularly polarized (RHCP), sequentially rotated hexagonal microstrip antenna array, working at GPS L1 band, is proposed. The proposed antenna design is placed on the hexagonal top of a floating sensor node for ocean and marine monitoring applications. To use the hexagonal area effectively and maximize the AP, CP microstrip antenna elements have been distributed centrosymmetric about the origin on the hexagonal substrate. Typically, the AP is enhanced by using thicker substrates, particularly with air gaps. Here, we show a high AP on a very low-profile substrate (0.011 for the lowest operating frequency) by properly choosing the distance of the antenna elements from the center of the array. The proposed design, based on H-shaped aperture-coupled circular patches, demonstrates a measured gain of 12.4 dBi and an AP of 101%, which is one of the highest reported AP for one of the lowest profile sequentially rotational array.