Body motions required for postural control while standing on a marine craft have not yet been characterized despite the popularity of such vehicles. It also remains unclear whether exercise loads for such postural control are greater than those of activities of daily living. The purpose of this study was, therefore, to investigate the kinematic characteristics and exercise load of passengers’ postural control against ship motion. We hypothesized that the exercise load of postural control while standing on a marine craft is greater than that while standing on land, which requires more rotations of the joints to maintain upright stability. In this paper, we report passengers’ standing postural motion, passengers’ exercise load and compared with other exercise loads on land. The measurement system consisted of three orientation sensors, a calorimeter, and a patient monitor. Orientation sensors capable of measuring linear and angular accelerations were placed on the head and waist of each participant, and on the floor of a small marine craft. Human energy expenditure was measured using the calorimeter. In the small marine craft, participants were either sitting or standing. In the laboratory, participants were sitting, standing, or performing step tests at 30, 40 and 50 steps/min. Standing postural motion against ship motion was analyzed by calculating the root mean square (RMS). On the marine craft, standing postural motions of participants were compared with the motions of a vertical standing rod to show the kinematic characteristics of the standing postural motion. Exercise load was calculated by dividing the 5-min mean energy expenditure in each condition by that in the sitting condition. Passengers’ standing postural motion on the craft was mainly rotational motion. Passengers’ exercise load increased with an increase in floor heave motion. The exercise load during postural control against ship motion was greater than that during quiet standing but was similar to that during stepping on land, with fairly low exercise intensity. We thus found that passengers had an exercise load attributable to postural motion.