There are 114 available titles in the game, but the level commendations for the three factions don't seem to count for more than 1 each and the level accommodations count. The 'Sailor' title is available by default. You have to both unlock the commendations and buy them in the clothes shop (they cost 0 gold) for then to count towards this achievement.
This is the full list of available titles as of February 2019. I've bolded 15 that are easy and/or fast to get, in my opinion. For a lot of these, there are achievements tied them (I marked them with *), so you'll be getting those titles anyway when going for the achievements.
Base game:
- Sailor
- Gold Hoarder level titles
- Order of Souls level titles
- Merchant level titles
- Pirate Legend* - Reputation level 50 on all three companies
Gold Hoarder titles:
- Golden Voyager* - Complete 250 Gold Hoarders voyages
- Keeper of a Glittering Hoard* - Earn 150,000 gold with the Gold Hoarders
- Sailor of the Gold Horizon* - Sail 1000 nautical miles on Gold Hoarder voyages
- Seeker of Lost Maps* - Purchase 250 Gold Hoarder voyages
- Hoarder of the Castaway's Gold - Sell 90 Castaway's chests to the Gold Hoarders
- Hoarder of the Seafarer's Gold - Sell 180 Seafarer's chests to the Gold Hoarders
- Hoarder of the Marauder's Gold - Sell 270 Marauder's chests to the Gold Hoarders
- Hoarder of the Captain's Gold* - Sell 360 Captain's chests to the Gold Hoarders
- Hoarder of Grog Soaked Gold - Sell 100 Chests of a Thousand Grogs to the Gold Hoarders
- Hoarder of Treasured Tears - Sell 100 Chests of Sorrow to the Gold Hoarders
- Hoarder of Barnacled Gold* - Sell 300 Shipwrecked Chests to the Gold Hoarders
- Hoarder of Lost Artefacts - Sell 450 Treasure Artefacts to the Gold Hoarders
Order of Souls titles:
- Voyager of Lost Souls* - Complete 250 Order of Souls voyages
- Mercenary of the Ancient Order* - Earn 150,000 gold with the Order of Souls
- Sailor of the Whispering Bones* - Sail 1000 nautical miles on Order of Souls bounties
- Seeker of Lost Souls* - Purchase 250 Bounty voyages
- Hunter of Foul Skulls - Deliver 70 Foul Bounty Skulls to the Order of Souls
- Hunter of Disgraced Skulls - Deliver 180 Disgraced Bounty Skulls to the Order of Souls
- Hunter of Hateful Skulls - Deliver 270 Hateful Bounty Skulls to the Order of Souls
- Hunter of Villainous Skulls* - Deliver 360 Villainous Bounty Skulls to the Order of Souls
- Hunter of Cursed Crews - Defeat X Skeleton Crews
- Hunter of Cursed Captains - Defeat 1000 Skeleton Captains
- Raider of Cursed Strongholds - Clear 120 Skeleton Forts
- Hunter of Fort Skulls - Deliver 60 Skeleton Fort Skulls to the Order of Souls
Merchant titles:
- Merchant Voyager* - Complete 250 Merchant voyages
- Gilded Merchant* - Earn 150,000 gold with the Merchant Alliance
- Sailor of the Merchant Alliance* - Sail 1000 nautical miles on Merchant Alliance business
- Merchant Adventurer* - Purchase 250 Merchant voyages
- Merchant Forager* - Deliver 50 Banana Crates on schedule
- Merchant Shipwright - Deliver 50 Wood Crates on schedule
- Cast Iron Merchant - Deliver 50 Cannonball Crates on schedule
- Black Powder Merchant* - Deliver 100 Gunpowder Kegs on schedule
- Merchant of the Dawn Caller - Deliver 200 Chickens on schedule
- Merchant of the Wild Hog - Deliver 200 Pigs on schedule
- Merchant of the Serpant's Scale - Deliver 200 Snakes on schedule
- Merchant of Grand Fauna - Deliver 200 Golden Animals on Schedule
- Merchant of the Silken Trade - Deliver 50 Crates of Luxurious Cloth on time and in perfect condition
- Merchant of the Flora Trade - Deliver 50 Crates of Plants on time and in perfect condition
- Merchant Rum Runner - Deliver 50 Crates of Rum on time and in perfect condition
- Merchant of Forsaken Silk - Deliver 50 Crates of Luxurious Devil's Cloth on time and in perfect condition
- Merchant of Forsaken Flora - Deliver 50 Crates of Devil's Plants on time and in perfect condition
- Merchant of Forsaken Rum - Deliver 50 Crates of Devil's Rum on time and in perfect condition
Athena's Fortune titles (must be Pirate Legend):
- Seeker of Athena's Fortune - Purchase X Athena's Fortune voyages
- Sailor of Athena's Fortune - Sail 1000 nautical miles on Athena's Fortune voyages
- Athena's Voyager - Complete 30 Athena's Fortune voyages
- Legendary Treasure Hunter - Sell X Chest of Legends to the Mysterious Stranger
- Legendary Thief - Sell X Chest of Legends to the Mysterious Stranger that were stolen
- Hoarder of Athena's Fortune - Earn X Gold from Chests of Legends
- Legend of Black Powder - Sell X Stronghold Gunpowder Barrels as a Pirate Legend
- Golden Legend of the Strongholds - Sell X Stronghold Chests as a Pirate Legend
- Mystical Legend of the Strongholds - Sell X Stronghold Skulls as a Pirate Legend
- Legendary Merchant of Bone - Sell X Crates of Ancient Bone Dust as a Pirate Legend
- Legendary Gold Hoarder - Sell 500 Chests as a Pirate Legend
- Legendary Merchant of Fauna - Deliver X Animals on time, as a Pirate Legend
- Legend of the Mystical Order - Sell 500 Bounty Skulls as a Pirate Legend
- Legendary Merchant Trader - Deliver 500 Cargo Crates on time and in perfect condition, as a Pirate Legend
- Legendary Hunter of The Hungering One - Defeat The Hungering One X times as a Pirate Legend
- Legendary Hunter of The Crested Queen - Defeat The Crested Queen X times as a Pirate Legend
- Legendary Hunter of The Shadowmaw - Defeat The Shadowmaw X times as a Pirate Legend
- Legendary Hunter of The Ancient Terror - Defeat The Ancient Terror X times as a Pirate Legend
- Legendary Hunter of the Shrouded Ghost - Defeat The Shrouded Ghost X times as a Pirate Legend
- Legend of The Sea of Thieves - Defeat 500 Skeleton Ships as a Pirate Legend
Festival of the Damned event:
- Legend of the Damned - Find and light all beacons around the Sea of Thieves with a Flame of Fate
Sunken Curse update:
- Curse Breaker - destroy 3 sapphire, 3 emerald and 3 ruby mermaid statues
- Master Curse Breaker - destroy 15 sapphire, 15 emerald and 15 ruby mermaid statues
- Legendary Curse Breaker - destroy any 100 mermaid statues
Cursed Crews update:
- Bearer of the Reaper's Mark - earn all of the cursed cannonball commendations against ships while flying the Reaper's Mark flag
- Legend of Cursed Iron Title - earn all of the cursed cannonball commendations against skeletons
- Legendary Cursed Voyager Title - complete 30 voyages while flying the Reaper's Mark flag
Shrouded Spoils update:
- Master Kraken Hunter - defeat 5 Krakens (*achievement for killing one)
- Legendary Kraken Hunter - defeat 10 Krakens
- Hunter of the Hungering One - defeat the Hungering One
- Hunter of the Crested Queen - defeat the Crested Queen
- Hunter of the Shadowmaw - defeat the Shadowmaw
- Hunter of the Ancient Terror - defeat the Ancient Terror
- Hunter of the Shrouded Ghost - defeat the Shrouded Ghost
- Master of Stronghold Spoils - sell 10 stronghold gunpowder barrels, 10 stronghold chests, 10 stronghold skulls, 10 crates of ancient bone dust
- Golden Skeleton Captain - sell 20 skeleton captain's chests
- Mystical Skeleton Captain - sell 20 skeleton captain's skulls
- Hoarder of Mermaid Treasure - sell 10 sapphire, 10 emerald and 10 ruby Mermaid gems
Gunpowder Skeletons update:
- Skeleton Exploder - kill 3 bone skeletons, 3 plant skeletons, 3 gold skeletons or 3 shadow skeletons by shooting a gunpowder skeleton, 3 times
- Master Skeleton Exploder - kill 3 bone skeletons, 3 plant skeletons, 3 gold skeletons or 3 shadow skeletons by shooting a gunpowder skeleton, 10 times
Skeleton Thrones update:
- Hero of Thrones - find and sit in the 5 small skeleton thrones
- Master of Thrones - find and sit in the 5 small skeleton thrones and the 5 large skeleton thrones (requires 2 crews)
Forsaken Shores update:
- Devil's Cartographer - discover all 15 devil's roar islands
- Master Devil's Voyager - complete 5 DR voyages for the gold hoarders and 5 DR voyages for the order of souls
- Merchant of Forsaken silk - deliver 50 crates of luxurious devil's cloth on time and in perfect condition
- Merchant of Forsaken Flora - deliver 50 crates of Devil's Plants on time and in perfect condition
Cursed Sails update:
- Protector of the Wilds - Win 'the Final Stand of the Wilds'
- Protector of the Shores of Plenty - Win 'the Battle for the Shores of Plenty'
- Protector of the Ancient Isles - Win 'the Battles for the Ancient Isles'
- Conqueror of the Skeleton Fleets - Win all above battles
- Seeker of thet Warsmith - investigate Wanda's Workshop
Mercenary update:
- Loyal to the Coin - complete a Mercenary voyage (available from Duke in any tavern)