In this paper, we applied our soft sensorless gait assistive suit to underwater walking, which is popular as a training method for elder persons and persons in rehabilitation. The underwater training has an effect that the weight of the trainee is canceled by water's buoyancy, and the trainee can carry out desired walking exercises with lower leg load by using the gait assistive suit in the underwater situation. The gait assistive suit is driven by pneumatic artificial rubber muscles (PARMs). The walking phase is detected from the pressure derivative in the PARMs, and the suit assists walking during the swing phase. The gait assistive suit consists of an attachment unit and a control unit, and these units are separated physically and connected only by air tubes. The attachment unit has a simple and sensorless structure without any electrical devices. Sensors in the control unit are only pressure sensors, and pressure information is used for detecting appropriate assist timings. Therefore, even when the attachment unit is in the underwater, the proposed gait assistive suit is able to use in the same manner as the ground situation. To verify applicability of the proposed gait assistive suit, we carried out the underwater walking experiments. As a result, it was confirmed that the gait assistive suit worked correctly in the underwater situation.