Achievements are, as of this moment, a cool system that can be expanded upon so that players seek them out more actively. Why not unpack some ideas?
I. Each achievement gained grants 5 Diamonds to your account. That would also help underaged players gain diamonds without having to use their mom's credit card at 4am.
II. Certain boss-slaying achievements grant a unique title you can flaunt.
III. Certain boss-slaying achievements grant a a unique trophy you can place on your crew's ship.
IV. Grant a player rewards per threshold of achievements- for example, if a player reaches 5 achievements, reward them with a few Berry. When they reach 10, 15, 20, etc, repeat the process but slightly scale the Berry reward a reasonable amount to make sense for them to achievement-hunt, to give them incentive to play more.
V. Faction-specific achievements; Marines get offered a badge of service, badge of honour, whatever you wish to flavour it as, per threshold of pirates killed in PvP combat. They can then refund those badges for unique alternate uniform styles. On the other hand, Pirates get a hefty reward of Berry per equal threshold of marines killed in PvP combat.
VI. Grinding achievements grant a fixed number of Soul Energy. That way, even if you're unlucky during looting you can still get some payoff when the achievement notification hits the screen.