Many people like windsurfing------(in the sea ,on the sea)

Many people like windsurfing------(in the sea ,on the sea) i can see people who are windsurfing on the sea这句话对不对 in the sea还是on the sea On the sea 和in the sea I can see people windsurfing ___ the sea ___填什么,理由 fly in the sea还是fly on the sea? travel in the sea 还是 travel on the sea.. in the sea 和on the lake by the sea 于on the sea in the sea on the sea,at sea有什么区别 in the sea ,on the sea ,at sea 有什么区别, Do you like swimming in the sea?