In an episode in the tv series "I shouldn't be alive", crew members of a yacht racing team got caught in a violent storm and found themselves stranded in the Gulf of Mexico after their boat capsized. Drifting for two three days, they spotted an oil rig in a distance and decided to swim towards it. However, the ocean current that brought them near also carried them away.
Let's assume that the oil rig is of a distance 5km away and within the reach of an average strong swimmer under calm water condition. If the current speed is significant but not more than half the swimmer's average speed, there is a fair chance of reaching the rig provided that the swimmer is able to judge the current speed and direction and set his course in a direction such that the resultant path forms a straight line between his initial position and the rig itself.
Mathematically speaking, the resultant velocity (speed and direction) is the trigonometric sum of two velocity components: the current velocity and the swimmer's velocity. So my question is, how can a swimmer estimate the current velocity while floating in the sea.