As used for a voyage across the Atlantic? (8)

'atlantic?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'as used for a voyage across the' is the wordplay.
'as used for' becomes 'boa' (I can't justify this - if you can you should give a lot more credence to this answer).
'a voyage' becomes 'sail' (sail is a kind of voyage).
'across' says to put letters next to each other (across can mean on).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'boa' put after 'sail' is 'sailboa'.
'sailboa'+'t'='SAILBOAT'

Can you help me to learn more?