| Abbreviation | Definition |
|---|---|
| ULB | Underwater Light Beacon |
| ULB | Un-Lighted Buoy |
| ULF | Unidentified Life Form |
| ULR | Ultra Low Range |
| UMBSM | University Marine Biological Station, Millport |
| UMI | United Marine International |
| UMK-3 | Tactical Environmental Support System / Shipboard Meteorological and Oceanographic Observing System |
| UMP | Upper Mantle Project |
| UMV | Unmanned Work Vehicle System |
| UMWW | Unprocessed Municipal Waste Water |
| UNCED | United Nations Conference on Environment and Development |
| UNCLOS | United Nations Conference on the Law of the Sea |
| UNCLOS | United Nations Convention on the Law Of the Sea |
| UNCSTD | United Nations Commission on Science and Technology for Development |
| UNCSTD | United Nations Conference for Science and Technology for Development |
| UNDP | United Nations Development Program |
| UNEP | United Nations Environment Program |
| UNGA | United Nations General Assembly |
| UNIENET | United Nations International Emergency Network |
| UNIS | Unmanned Nautical Informational Submersible |