In Europa Universalis II, when you have military access in a country you can sail fleets into that country's ports. This is true even if you are currently at war. You can also, of course, sail your fleets into your own ports. If a fleet ends its move in an enemy port settlement, and the settlement is unfortified, the settlement is captured. It is captured regardless of whether it is occupied by land units; land and sea units do not interact, so there is no battle.
It's not clear why Paradox implemented things this way; perhaps they had some idea of fleets capturing ports even without military access, but never implemented code for a fleet to force its way into an enemy port. Except, of course, with military access. An oversight? Maybe.
In any case, this feature makes it pretty easy to deal with colonial AIs. They don't build forts very much in their colonial cities , and of course nobody can fortify a colony. So, get military access from Spain or whichever power you intend to attack. Place a galley in each of its unfortified ports. Then get into a war with it. You cannot declare war yourself (because of the military access). But you might declare war on one of its allies, or join an ongoing war against it, or get it to declare war on you. When war is initiated with a country, all fleets in its ports are sent to sea. But you can just cancel the move, and voilà! Captured settlement.
You have to cancel the move twice, typically. The first time captures the settlement, but re-orders the fleet to sea. The second time stops it.
You may want to sue for peace immediately, if all you want is to grab some colonies. If you want a long war, no problems. Just leave your galleys in his ports; they can't be attacked there, and do not suffer attrition because they are in port. If the enemy marches land units in, they'll retake the settlement; but then you just cancel the move to sea (twice), and it returns to your control, even though his troops are in the city!
Note that the same trick will work in your own unfortified ports. Just leave a galley there whenever you're in a war. If an enemy takes the province, immediately stop the move to sea (twice), and you retain control, and your galley is not forced out to sea.