In this paper, we proposed the concept of underwater soft exoskeleton which could provide assistance to swimming motions. With this motivation, we designed the first version prototype called Powered Swimsuit to assist the wearer in fin swimming. The actuator system was embedded inside of a portable water-proof cabin that was fabricated on the back of the swimsuit. The assistive force was applied on the bottom of the fins via soft cables. During the propelling period of the stroke cycle, the cables pulled the ankle joints to provide assistance to plantarflexions. After implementation of the prototype, we carried out initial experiment to test the proposed soft exoskeleton. The subject was asked to swim with frog kicks under two assistance levels. Several kinematic parameters including the tilt angles, accelerations and angular velocities of the lower extremities were recorded simultaneously. With higher assistance, larger absolute tilt angles and accelerations, larger signal profile changes in angular velocities were found within the sagittal plane of the measured foot. The preliminary results showed the feasibility of the proposed Powered Swimsuit in underwater motion assistance.