In this paper we present a sea vessel detection algorithm in aerial image sequences acquired by an unmanned aerial vehicle. The proposed method is robust to variable background lighting, highlights due to sun reflections, vehicle self motion and scale changes. By relying in simple blob analysis rules, based on both spatial and temporal constraints, the algorithm is capable of real-time operation onboard the vehicle, even with non optimized code. We evaluate our method on three sequences labeled with ground truth vessel position, with more that 2900 frames. Overall we are able to achieve very low false positive rates even in heavy sun reflection conditions.