According to British Phonetics Buoy has the same 'transcription' (i.e. the way you pronounce a word is the way you write it down) as Boy.
Buoy: /bɔɪ/
Boy: /bɔɪ/ (these signs are used to change sounds (phonemes) into readable words)
http://oxforddictionaries.com/definition/buoy
http://oxforddictionaries.com/definition/boy
American English shares the same pronunciation.
This is the British English Phonetic Chart; It shows all the phonemes you need. Look up how you should pronounce Buoy/Boy.

http://www.teachingenglish.org.uk/try/activities/phonemic-chart