| MARPOL | - | International Convention for the Prevention of Pollution from Ships |
| MBL | - | Minimum Breaking Load |
| MCA | - | Maritime and Coastguard Agency |
| MCO | - | Maximum Continuous Output |
| MCR | - | Maximum Continuous Rating |
| MDO | - | Marine Diesel Oil |
| MED | - | Mediterranean |
| MEES | - | Middle East Emergency Surcharge |
| MEPC | - | Maritime Environment Protection Committee |
| MERSAR | - | Merchant Ship Search and Rescue manual |
| MES | - | Mobile Earth Station |
| MFO | - | Marine Fuel Oil |
| MGO | - | Marine Gas Oil |
| MGW | - | Maximum Gross Weight |
| MHHW | - | Mean Higher High Water |
| MHHWS | - | Mean Higher High Water Spring |
| MHLW | - | Mean Higher Low Water |
| MHW | - | Mean High Water |
| MHWI | - | Mean High Water Interval |
| MHWN | - | Mean High Water Neaps |
| MHWS | - | Mean High Water Springs |
| MLB | - | Mini Land Bridge |
| MLD | - | Moulded |
| MLHW | - | Mean Lower High Water |
| MLLW | - | Mean Lower Low Water |
| MLLWS | - | Mean Lower Low Water Spring |
| MLW | - | Mean Low Water |
| MLWI | - | Mean Low Water lunitidal Interval |
| MLWN | - | Mean Low Water Neaps |
| MLWS | - | Mean Low Water Springs |
| MID | - | Maritime Identification Digits |
| MMSS | - | Maritime Mobile Satellite Service |
| MOB | - | Man Over Board |
| MODU | - | Mobile Offshore Drilling Unit |
| MOL | - | More Or Less |
| MOLCO | - | More Or Less Charterer's Option |
| MOLOO | - | More Or Less Owner's Option |
| MOT | - | Ministry Of Transport |
| M/R | - | Mate's Receipt |
| MS | - | Motor Ship |
| MSA | - | Marine Safety Agency |
| | - | Merchant Shipping Act |
| MSB | - | Main Switch Board |
| MSC | - | Maritime Safety Committee |
| MSG | - | Message |
| MSI | - | Maritime Safety Information |
| MSL | - | Mean Sea Level |
| M/T | - | Motor Tanker |
| MT | - | Motor Tanker |
| | - | Metric Ton |
| | - | Empty |
| MTBE | - | Metyl Tertiary Butyl Ether |
| MTC | - | Moment to change Trim per Centimetre |
| MTD | - | Multimodal Transport Document |
| MTL | - | Mean Tide Level |
| MTO | - | Multimodal Transport Operator |
| M/V | - | Motor Vessel |
| MV | - | Motor Vessel |
