Windward

You have to capture a level 9+ region with at least another player who meets the level requirement of the area as well (Tried it with an alt just idling in the area and didn't work).

Also keep in mind, this area must be controlled by PIRATES and NEVER WAS INVADED BEFORE!

I invaded a zone occupied by another zone, it didn't work. I also invaded a region already occupied by another faction, as a pirate. Afterwards invaded it with a friend as a faction member but again it didn't work, so don't waste your time!

Simply we joined to a fresh public server (wanted to be sure all zones were fresh and never were invaded before by anyone else), leveled up our starter ships to 15+ (that was the level of the neighbour zone we had) and invaded it to get the achievement. Keep in mind you always have to invade at least 1 neighbour zone before invading further zones, you can not simply rush for a random zone on the map so pick your server/map carefully according to your plan.

Easiest way to get this achievement is to go to a public server with a partner and select the same faction, therefore you can start the game on the same zone.

Once you find eachother on the map, one of you has to set your ship as a pirate ship (through the inventory screen, dropdown list for faction selection also has the pirate option) and you will be able to attack eachother. Once you sink your partner's ship, you will get the achievement.

This achievement requires only 12 ships to be owned, game has much more ships to be bought so the description is misleading and it is easier than it sounds to get the achievement.

Many of the ships are unlocked to be bought by sinking the same kind of ships in the open world. Try visiting different level zones to find different ships. The higher level zone has, the higher base-powered ships you will encounter.