Mariculture is a high‐risk industry that is susceptible to weather disasters. However, due to moral hazard, adverse selection and high transaction costs, traditional indemnity insurance policies are not available. An emerging alternative is the development of weather index mariculture insurance. This research assesses the effectiveness of weather index mariculture insurance by using the swimming crab precipitation index insurance as an example. The theoretical and empirical results suggest that weather index mariculture insurance is not always effective. It cannot be guaranteed to promote the welfare of mariculturists and reduce the tail risk of income, especially in the case of gross rates. However, in the case of low basis risk, it could be a viable option for the government because of the low subsidy cost required.