A ship including a gas re-vaporizing system and a seawater supply apparatus, the gas re-vaporizing system including a re-vaporizing apparatus, which re-vaporizes liquefied gas through seawater supplied by the seawater supply apparatus, wherein the seawater supply apparatus includes: a seawater supply line which supplies the seawater to the re-vaporizing apparatus; a seawater discharge line which discharges the seawater from the re-vaporizing apparatus; a circulation connection line which is branched from the seawater discharge line and connects to the seawater supply line, wherein an air removal valve is provided on the circulation connection line; a first opening/closing valve which is disposed in the circulation connection line closer to a point of connection with the seawater supply line; a second opening/closing valve which is disposed in the circulation connection line closer to a point of connection with the seawater discharge line; a seawater pump which is provided on the seawater supply line and supplies the seawater to the re-vaporizing apparatus; a third opening/closing valve which is provided in an upstream of the seawater pump on the seawater supply line; a fourth opening/closing valve which is provided in a downstream of a branch point of the circulation connection line in the seawater discharge line; a pressure maintaining fluid supply valve which is provided on a tank connection line; a third control unit which adjusts opening of the first to fourth opening/closing valves (B8, B3, B1, B2) and the pressure maintaining fluid supply valve, and implements a switch so that the seawater flowing in the seawater discharge line flows the circulation connection line without being discharged to the outside non-stop; a seawater storage tank which is provided on the circulation connection line, and maintains pressure of the seawater flowing in the circulation connection line; and wherein the tank connection line connects the seawater storage tank and the circulation connection line, and wherein the seawater pump is located below a sea surface, and the tank connection line closes the first opening/closing valve and the second opening/closing valve and supplies a fluid inside the seawater storage tank on the circulation connection line before the seawater flowing in the seawater discharge line is switched to flow the circulation connection line without being discharged to the outside. a seawater supply line which supplies the seawater to the re-vaporizing apparatus; a seawater discharge line which discharges the seawater from the re-vaporizing apparatus; a circulation connection line which is branched from the seawater discharge line and connects to the seawater supply line, wherein an air removal valve is provided on the circulation connection line; a first opening/closing valve which is disposed in the circulation connection line closer to a point of connection with the seawater supply line; a second opening/closing valve which is disposed in the circulation connection line closer to a point of connection with the seawater discharge line; a seawater pump which is provided on the seawater supply line and supplies the seawater to the re-vaporizing apparatus; a third opening/closing valve which is provided in an upstream of the seawater pump on the seawater supply line; a fourth opening/closing valve which is provided in a downstream of a branch point of the circulation connection line in the seawater discharge line; a pressure maintaining fluid supply valve which is provided on a tank connection line; a third control unit which adjusts opening of the first to fourth opening/closing valves (B8, B3, B1, B2) and the pressure maintaining fluid supply valve, and implements a switch so that the seawater flowing in the seawater discharge line flows the circulation connection line without being discharged to the outside non-stop; a seawater storage tank which is provided on the circulation connection line, and maintains pressure of the seawater flowing in the circulation connection line; and wherein the tank connection line connects the seawater storage tank and the circulation connection line, and wherein the seawater pump is located below a sea surface, and the tank connection line closes the first opening/closing valve and the second opening/closing valve and supplies a fluid inside the seawater storage tank on the circulation connection line before the seawater flowing in the seawater discharge line is switched to flow the circulation connection line without being discharged to the outside.