by sea, by the sea, in the sea, on the sea, at sea
(1) by sea “走海路，乘船”，用来表示交通方式，同 by ship 同义。
These heavy boxes should be sent by sea.
(2) by the sea “在海边”，相当于 by (at) the seaside。
The children enjoyed themselves by the sea on Children's Day.
(3) in the sea “在海里，在海水中”
There are many plants and animals in the sea.
(4) on the sea “在海面上”，“在海岸边”。
I want to live in a town with a beautiful position on the sea.
(5) at sea 在海上;在航海
When he woke up, the ship was at sea.