Underwater wearable sensors utilizing conductive hydrogels have garnered significant attention in recent years. However, the response sensitivity to the mechanical strain, quantified by the gauge factor (GF), of most hydrogels is noticeably diminished when submerged in water, and little consideration has been given to the GF value of sensors operating both in air and underwater. Consequently, the development of underwater sensors with high sensitivity in aquatic environments remains a challenge. In this study, we propose a “soaking-in-water” strategy to enhance the sensitivity of the wearable sensor based on starch/poly(vinyl alcohol)/graphene oxide/ionic liquid hydrogel. Through this approach, the maximum GF of the hydrogel underwater was improved to 9.71, representing an 86.7% increase compared to the unsoaked hydrogel (GF of 5.20). Furthermore, the hydrogel demonstrated adjustable conductivity (from 0.26 to 1.82 S·m⁻¹) and tensile properties (from 0.05 MPa at 244% to 0.21 MPa at 527%). The hydrogel underwent the processes of water-absorbing swelling, exudation of ionic liquid and water-repelling shrinkage. The enhancement in sensitivity and swelling mechanism of the hydrogel were closely linked to the movement of ions and water between the hydrogel and soaking water. Leveraging these properties, we further developed an underwater strain sensor capable of monitoring human motions underwater, offering quick, effective, and stable signal transmission. The proposed soaking method represents a promising avenue for improving the sensitivity of hydrogel sensors, providing a facile strategy for achieving accurate and efficient underwater monitoring applications.