Cable conductor temperature is an important parameter to determine the cable ampacity. On the other hand, the surrounding condition has a tremendous influence on cable conductor temperature. In order to improve the operating current of cable, this article derivates the calculation formulae for the ampacity of single-core AC cable laid in water, by analyzing the operating current characteristics of the single-core AC cable laid on water. The further correctness of the theoretical analysis is demonstrated by the comparison of the experimental statistics of conductor temperature of the single-core AC cables respectively immersed in water and in air without being exposed to the sunlight. It also draw a conclusion that AC cable laid on water can maximize the cable ampacity, and provide important theoretical basis to maximize the cable ampacity.