Can't say I'm too surprised by this policy. I had a whole-day meeting with a colleague from another local authority, and suggested we nip over to the pub for a sandwich at lunchtime. I might as well have announced my intention to take a dump on the carpet - they have a strict zero tolerance policy on alcohol during working hours, and that's just an office block, arguably a somewhat safer environment than a ship!
Going back to the OP, can you go ashore and have a sherbet or is that also beyond the pale?
As an aside, I am currently sipping a ghost hunter, ie equal parts Southern Comfort and bourbon over ice.