I went through the same exercise and was given the actual monthly expenses by a current boat owner in DB marina.
Monthly expense for a 55' boat, inclusive of all the marina fees etc, (as well as your annual prepaid insurance and annual hull scrape /12) comes to about $26500.
Then you have the pending decision of the DB marina to re-evaluate the debenture cost in 2018. You will be liable for the increased debenture cost. As to date, there is no indication on how much it will rise. Care to venture a guess?
HKIRD now also prohibit any domestic helpers to live on your boat, and it's illegal to let her live elsewhere.