sea

My brother is a sailor and spends most of the year in the sea.
My brother is a sailor and spends most of the year at sea.

Use at sea when someone is sailing, travelling or working on a ship: ‘My father spent ten years at sea.’

Use the preposition in when someone or something is inside the water: ‘The children are swimming in the sea.’ ‘There are many fish in the sea.’

Use by the sea to mean ‘near the sea’: ‘They live in a small town by the sea.’