Here is the full list of all 30 Blue Horizon achievements.
Smash 50 Barrels you find
Defend Port Cutlass from the Pirate Ship using the Cannons
Escape Dead Mann's Island by Building a Raft and Sailing to Port Cutlass
Build a Raft on Dead Mann's Island from Parts Found
Complete the Entire Game
Defend Fort Williamson from the Pirate Fleet using the Cannons
Defeat the Stone Warrirors and get the Cursed Golden Mask
Defeat the Crystal Guardians in the Crystal Cavern
Find and talk to Mermaids near the shore
Tame the Sea Beast in the Ice Underworld
Smash 50 Crates you find
Kill 100 Small Crabs you find
Kill 100 Flamingos you find
Kill 5 Sharks in the Sea
Sink 5 Small Red Sailed Pirate Ships with your Cannons
Sink 4 Triple Masted Galleons with your Cannons
Kill 5 Giants Crabs you find
Give all the Blue Sea Pearls to the Mermaids on the Shore
Get the MerKing Trident
Gaol Break Guy Silver and charter the Horizon
Find the Pegleg in the Graveyard and Open It
Achieve a Bounty on the Head of the Pirates Moored
Smash open 50 Coffins you find
Kill 5 members of the Death Skull Tribe
Kill 20 Snakes you find
Kill 10 Giant Rats you find
Find 10 Hidden Treasure Chests Stashes on and under the Islands
Kill 10 Crocodiles
Free yourself of the Black Spot
Get all the Blue Sea Pearls from Smugglers Rock