in the sea 和on the lake

in the sea 和on the lake in the lake和on the lake On the sea 和in the sea in the sea还是on the sea in the lake 与 on the lake 什么差别? boat和ship分别是in the sea 还是on the sea at fly in the sea还是fly on the sea? Many people like windsurfing------(in the sea ,on the sea) travel in the sea 还是 travel on the sea.. On land as in 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,