Drag is the transfer of momentum from our moving object to a fluid. A crew moves through both water (in contact with the shell) and air (in contact with a small part of the shell and the rowers).
To minimize air resistance rowers wear skimpy unis and force their coxswains to lay in the very short and narrow stern of the boat. This is less of a problem than the drag through water because water is the more viscous fluid.
Most boats are slowed by primarily wave drag however, racing shells are unique in that skin drag is the primary resistive force. Dudhia claims 80% of the resistance for shells is due to skin drag. If only taking into account skin drag the resistance of the boat is proportional to the velocity of the boat (Equation 1).