A boat includes a connection structure that connects couplings via a transfer shaft on a rear end portion of a crankshaft extending rearward from an engine. The crankshaft and the transfer shaft are connected by screwing an external thread and a front internal thread, and the transfer shaft and the couplings are connected by screwing an external thread and an internal thread. There are provided an inner bolt that strengthens the connection between the external thread and the front internal thread, an end bolt that strengthens the connection between the transfer shaft and the couplings, and an outer nut that strengthens the connection between the crankshaft and the transfer shaft. The boat can prevent movement between the crankshaft and an intermediate member and between the intermediate member and a coupling both in a rotational direction and in an axial direction without causing an increase of processing time or processing cost.