I am actually not 100% sure what the procedure in Malta actually is regarding paquebot. In this case there's an actual letterbox on board the vessel - I don't know if mail from other vessels is also accepted and sent as paquebot.
MaltaPost began the paquebot service on board the MV
Jean de La Valette on 7 November 2011. The announcement on their website reads:
Quote:
MaltaPost Service on board M.V. Jean De La Valette
04/11/2011
MaltaPost announces that it has installed a letterbox on board the M.V. Jean De La Valette. Mail posted on board the vessel will be marked "PAQUEBOT" and postage stamps cancelled accordingly. Mail posted during voyages to Malta will be collected and processed upon arrival at Malta.
Service of this letterbox will commence on Monday 7th November 2011.
Source:
https://www.maltapost.com/news-details?id=94Here is a pic of the letterbox:

This notice is affixed to the front of the letterbox:
Quote:
Articles posted in this mailbox on sailing days will be collected and processed upon arrival at Malta.

On the sides of the letterbox there are two identical notices which read:
Quote:
In terms of Universal Postal Union (UPU) regulations, a postal article bearing correct stamps and posted in this mailbox during the voyage to Malta shall be delivered to the addressee accordingly.

I believe the "MAILED ON BOARD..." marking is applied on board the vessel (or by the company owning the vessel). The "<PAQUEBOT>" marking and the "PAQUEBOT/MALTA" cancel are applied by MaltaPost on Malta after the mail is handed over to them. Mail is not collected very frequently. I posted mine on 22 December and they were postmarked on 7 January.
To be honest, I don't know why the service was put into practice. Having a paquebot service on board a ferry which makes daily trips is quite unnecessary in my opinion - it wouldn't have made much sense a hundred years ago, let alone in 2019. It's not philatelic either - I don't know of any way to send covers from there, except to physically post them at the letterbox on board.
The company operating the
Jean de La Valette, Virtu Ferries, had other catamarans before this one entered service in 2010. I don't know if they had similar arrangements with a letterbox on board - I haven't seen any press releases or anything and I've never seen any covers. Recently a new catamaran (the MV
Saint John Paul II) was launched and it is supposed to enter service later this year - it would be interesting to see if a letterbox is installed on this one too.