You need a fleet of about 30 ships, and a enemy country with at least 10 reachable coast provinces. if you want to get this quick England starts with a large fleet, most other countries needs to build it which require money and years of construction. it's then just about picking the right enemy. preferably one with a lot of province harbors in the same water province. ie so you don't need to split your fleet as much which can be dangerous in a war. Also your fleet get attrition far away from your home country so either you pick a close enemy or you do this achievement fast. England VS Ottomans or Mamluks would require a fast strategy, Ottomans barely starts with any fleet. Remember that you need to start the war before you can board your cogs with armies, if you intend to win the war, which isn't necessary.
Personally I did it with Ottomans where I blockaded the Mamluks. (after sinking most of their fleet) but it took some time to build that fleet. I recommend doing it that way and go for other achievement at the same time. Ottomans give you a good starting point for multiple achievements.