I was ready for the Shipmaster achievement already some time ago - but I kept my fleet size on 7 ships on purpose just for days like today when I don't have much time to play. So today I launched the console, went to my naval map, bought the 8th ship and got the achievement in less than 3 minutes. :)
My suggestions on getting this achievement:
- start building your fleet early (you can do this with any ships, but I decided to do it slowly and only bought the biggest ships available).
- do not spend your money in the beginning on watches and outfits, use it primarily to buy more ships so you can make more money and more profit. If you think you have enough money, go for the weapons & pouches so you can carry more "ammo".
- check your fleet progress on regular basis, maybe even after every mission (sidemission).
NB: The fleet is only travelling when you are playing the game - so if you send your fleet out and close your console few minutes after that, the fleet has not arrived when you log back in much later time (in some other AC series games fleet will travel when you sleep or are playing some other game).