Wow what a lot of pathetic responses to this very good question. I'm a native English speaker and can get on fine in English. I know about 10 words of Cantonese and have been here 9 years and live in the NT. So ignore anyone who says canto is essential. It's not. However you do need "something" to supplement English to get stuff done. I have used my Limited Mandarin from time to time and always been understood. Mostly though I use my mobile phone and bring up pictures or words and point. Body language will also take you a long way... especially done with a smile. I have mainland staff who use Mandarin and it seems to be well understood. I liken canto vs Mandarin to French vs Italian. If you speak and read one it takes you a long way in the other.
Welcome to HK.