Here's a first pass at improved random encounter tables. They pretty much just remove the boring stuff and expand the existing entries' ranges to fill out the table.
Open Water Encounters (Levels 1-4)
01-14 1 giant octopus, which attacks the ship’s hull
15-34 1d4 sahuagin
34-37 1d4 koalinth
38-45 1d4 locathahs
46-51 1d3 harpies
52-54 2 merrow
55-57 1 sahuagin priestess and 1d4 sahuagin
58-59 1 koalinth sergeant and 2d4 koalinth
60-64 1d3 sea hags
65-67 1d4 blood hawks
68-70 1 sahuagin champion and 1d4 sahuagin
71-80 A ghost ship (4d4+4 skeletons, 1d3 specters, 1 wraith)
81-90 A pirate ship (4d4+4 bandits, 1d4 bandit captains)
91-99 Sahuagin raiding party (1 high priestess, 2 wave shapers, 2 champions, and 4d4 sahuagin)
00 1 young dragon (good or evil)
Open Water Encounters (Levels 5-10)
01-25 1 sahuagin high priestess and 1 sahuagin wave shaper
26-40 1d4 water elementals
41-47 1 harpy matriarch and 1d4 harpies
48-49 2d4 merrow
50-52 1 sahuagin baron and 1d4 sahuagin
53-57 1 koalinth sergeant and 2d4 koalinth
58-565 1d4 sahuagin blademasters
66-69 1 hydra
70 1 marid (genie)
71-80 A ghost ship (4d4+4 skeletons, 2d4 ghasts, 1d3 wraiths)
81-90 A pirate ship (4d4+4 bandit captains, 1d4 veterans, 1 mage)
91-99 Sahuagin raiding party (1 baron, 1 high priestess, 2 wave shapers, 3 champions, and 4d4+4 sahuagin)
00 1 adult dragon (good or evil)
Open Water Encounters (Levels 11-20)
01-18 1 sahuagin high priestess and 2d4 sahuagin champions
19-25 1 sahuagin baron and 1d4 sahuagin blademasters
26-39 A rank 3 whirlpool connected to the Elemental Plane of Water (p. 206)
40-43 A rank 4 whirlpool connected to the Elemental Plane of Water (p. 206)
44-45 1d3 hydras
46-48 1d4 koalinth sergeants and 3d10 koalinths
49-53 3d6 merrow
54-57 1 harpy matriarch and 2d8 harpies
58-60 1 dragon turtle
61-64 1 adult dragon (good or evil)
65-70 Mysterious island
71-80 A ghost ship (4d4+4 phantom warriors (Curse of Strahd), 1d4 wraiths, 1 sword wraith commander (Tome of Foes))
81-90 A pirate ship (4d6+4 bandit captains, 1d4+1 veterans, 1d2 mages)
91-99 Sahuagin raiding party (1 baron, 1 high priestess, 2 wave shapers, 3 champions, 2 blademasters, and 4d4+10 sahuagin, all riding shell sharks)
00 1 kraken