How do you make easy money in Assassin's Creed 4?

I found most of my money comes from naval warfare, but if that is too difficult for you, do treasure hunts with your maps or find chests.

Don't go into any naval battle against groups, even if your ship is well upgraded. fighting a bunch of Man'o'Wars is still gonna hurt.. you are only a glorified brig after all.

Always ALWAYS board ships, not only can you fix your ship up afterwards (or clean your wanted slate), but you only get half of the prize if you sink your enemy.

Something I recently learned and beat myself up over for not doing sooner is use your swivels every possible chance. I don't know what console your using, but on PC I didn't even know how to use the swivels until late in the game (holding the button wouldnt shoot them, and firing shot heavy shot instead of the swivel). It's simply just hold and release once it locks on... Upgrade them to the max level, and all you have to do is hold down the button to melt down every crit point and do a lot of damage.

If you go to taverns and "pay for information" or whatever it is, sometimes they will tell you the whereabouts of a naval convoy. the main ship on those is worth quite a bit of reales, so go for them any time you feel comfortable.

Sell resources you dont use. Cloth has very little use once you upgrade your storage areas and get the sail you want... so just start selling any extra you get. Whaling/hunting is also a quick relatively easy few hundred reales.