| 1. una rig |  | 76. underwater camera |
| 2. una-rigged boardboat | | 77. underwater concrete |
| 3. unattended light | | 78. underwater cutting |
| 4. unbalanced laminate | | 79. underwater cutting torch |
| 5. unbalanced rudder | | 80. underwater cutting torch lighter |
| 6. unbalanced weave | | 81. underwater explosion |
| 7. unbalanced woven rovings | | 82. underwater hull |
| 8. unballast | | 83. underwater lamp |
| 9. unbend | | 84. underwater range |
| 10. unberthed passenger | | 85. underwater tools |
| 11. unberthed ship | | 86. underwater torpedo tube |
| 12. unbitt | | 87. underwater valve |
| 13. uncapsizing ship | | 88. underwater vehicle |
| 14. uncharted | | 89. underwater welding |
| 15. UNCLOS | | 90. underwater-to-air missile |
| 16. uncoil | | 91. underwater-to-surface missile |
| 17. uncouple motions | | 92. underwater-torch |
| 18. uncovered | | 93. underway replenishment tanker |
| 19. undamped motion | | 94. underwrite |
| 20. undecked | | 95. underwriter |
| 21. under (strict) supervision | | 96. underwriting |
| 22. under bare poles | | 97. undisturbed flow |
| 23. under bevel | | 98. undo a nut |
| 24. under canvas | | 99. undock |
| 25. under construction | | 100. undulate |
| 26. under current | | 101. undulation |
| 27. under deck tonnage | | 102. undulatory |
| 28. under easy sail | | 103. unequal-angle bar |
| 29. under full sail | | 104. unfair holes |
| 30. under hatches | | 105. unfavourable winds |
| 31. under load | | 106. unfurl |
| 32. under repair | | 107. unhook |
| 33. under sail | | 108. unicellular foam (plastic) |
| 34. under sailing orders | | 109. unidentifiable cargo |
| 35. under ship's tockle | | 110. unidirectional (reinforced) composite |
| 36. under stress of weather | | 111. unidirectional cloth |
| 37. under the lee | | 112. uniform general cargo |
| 38. under the wind | | 113. uniform pitch |
| 39. under way | | 114. uniform system of buoyage |
| 40. under weigh | | 115. uniform time signal |
| 41. under-deck | | 116. uniform time system |
| 42. under-keel allowance | | 117. uniform turning motion |
| 43. underbid | | 118. Unilock |
| 44. undercamassed | | 119. union |
| 45. undercarrier | | 120. Union Flag |
| 46. undercharge | | 121. union gear |
| 47. undercoat | | 122. Union Jack |
| 48. undercoating | | 123. union purchase rig |
| 49. underdeck cargo | | 124. union purchase swivels |
| 50. underfoot | | 125. union stockless anchor |
| 51. underhand | | 126. union system |
| 52. underhaul | | 127. union thimble |
| 53. underhung rudder | | 128. unit |
| 54. underinch | | 129. unit load |
| 55. underkeel clearance | | 130. unit of account |
| 56. underloading | | 131. unit patrol |
| 57. undermanned | | 132. unit power |
| 58. underpowered | | 133. United Nations Convention on the Law of the Sea |
| 59. underquote | | 134. unitized cargo |
| 60. underrun | | 135. Universal fairlead |
| 61. underrun (a rope) | | 136. universal joint (coupling) |
| 62. underrun a net | | 137. universal rule |
| 63. underrun a tackle | | 138. universal time |
| 64. underrunning | | 139. unkinkable lay |
| 65. undersail | | 140. unknown destination |
| 66. underset | | 141. unlash |
| 67. undersized fish | | 142. unlay |
| 68. understaffed | | 143. unlit |
| 69. undertaking | | 144. unload |
| 70. undertone | | 145. unloading deck |
| 71. undertone index | | 146. unmanned |
| 72. undertow | | 147. unmanned submersible submarine |
| 73. underwater | | 148. unmarked |
| 74. underwater body | | 149. unnamed |
| 75. underwater burst | | 150. unreliable sector |
