It is a pretty simple achievement to get. From the ware house after you unlocked the ability to send ships for trades, go into any "dock" and there is option to buy ship. You only need to purchase 7, so you own 8. Once you purchase the 7th ship, the achievement will pop.
TIP: When you click on the "buy ship", there is a price at the top for the unit, there are 4 different units available and the cheapest one is 3700 per unit. Don't spend all of your currency since the watches get expensive for the collection achievement.