Ask Reception to inform the Ship's Purser you want to speak to him/her. we did and we got a note asking us to meet her and explain the problem .
Persons who wish to complain about a safety-related matter they have observed on a cruise ship should contact the Coast Guard Marine Safety Office (MSO) responsible for the Control Verification Examination of the ship. Interested persons can get a referral to the appropriate MSO by calling the Coast Guard toll-free Consumer Hotline: 1-800-368-5647. Sorry can't so far get a better contact telephone line.
I so far have failed to find a more direct contact telephone number but I should have said the contact I did supply would only apply if it were an American cruise line but not many are not now with Carnival owning so many including Cunard. If it was a Britsih based cruise line and there won't be many then it would be our Coastguard authority as I am assuming this is something that has happened on a cruise ship you may have been on recently.
You could try:
Maritime and Coastguard Agency
Spring Place 105 Commercial Road
Southampton SO15 1EG
telefax: +44 23 80 32 91 04
telephone: +44 23 80 32 93 43
I hope that helps.