Any insurer who delivers or issues for delivery in the state any policy, other than homeowners, covering a motorboat used for recreational purposes and powered by a motor in excess of five horsepower shall allow a rate credit when the principal operator or operators has submitted to such insurer proof of successful completion of a course in safe boat handling approved by the Commissioner of Energy and Environmental Protection in accordance with § 15-133c or the provisions of § 15-133b. For policies delivered or issued for delivery on or after January 1, 1983, and before January 1, 1986, the credit shall be not less than five per cent of the premium for a period of two years from the effective date of the policy.