Here is the full list of all 28 Maelstrom achievements.
Ram an enemy ship and sink it to win the match
Sink an enemy ship while at 350 speed or higher
Win the game while in Dead Waters
Sink 2 enemy ships within 5 seconds of each other
Collect at least 5 powerups in dead waters without sinking
Sink 2 enemy ships with the same volley
Sink the last 5 enemy ships and win
Use boarding hooks to grab a powerup moments before an enemy ship grabs it
Win the game with the last enemy ship being sunk by hazards or sea monsters
Damage 6 enemy ships without sinking them and win the game
Deal 9,000 damage against enemy ships in one game
Deal 11,000 damage against enemy ships in one game
Deal 13,000 damage against enemy ships in one game
Collect 6 bounties in one game
Collect 8 bounties in one game
Collect 10 bounties in one game
Collect 6000 Gold in one game
Collect 8000 Gold in one game
Collect 10000 Gold in one game
Sink 3 enemy ships with ramming in one game
Sink 3 enemy ships with boarding in one game
Sink an enemy ship while not detected by them (e.g. in Fog)
Sink an enemy ship by causing them to sink in a Maelstrom
Sink an enemy ship while having less than 10% Hull remaining
Sink two enemy ships while under the influence of the same Repair Boon
Collect 8 powerups dropped by enemy ships you or your team didn't kill
Absorb at least 10,000 damage using armor or shields
Receive at least 8,000 damage in a match and win