Keeping Track of the Enemy
It is a good idea to "set sail" to the "potential enemy" Haven so ye can view it from the inside. Record the captain's level and current experience points in yer logbook. After 3-7 days start goin' back and checkin' on them. Those that do not move in experience points are usually inactive and safe to raid without retaliation.
Noting down the Harbor Level
The other part once ye found it then send a Flyin' Dutchman to scout it right before your raid hits. Take the booty and subtract it from the totals - this will tell ye the harbor protection value. Make a note of this amount so in the future a scout will tell ye how profitable the raid will be. Also keep a scout or a blockade in/on the Haven which can let ye know if others have raided it and when so ye can track their patterns and get there first.
Identifying Yer Regular Targets and Inactive Pirates vs. Active Pirates
There are many inactive Havens level 17-25 that will give ye 3k lumber and 3k gold. I even found a level 9 that nets 2.5k of each and less than 1k of rum. Me find the lower level ones to be less competition from other captains for the resource and ye can raid them all week rather than gettin' say 10k of resource and 40k of rum then waitin' a week fightin' off other cap'ns to keep it.
I prefer inactives to actives...there be more profit. You don't have to worry about them hittin' yer ransom 'cades, lighthouse targets, or returnin' the favor in ye sleep. I have not lost troops raidin' dead havens either. I know some cap'ns see the experience points as a win but I can raid inactive low levels all day long to gain experience without losin' a man.
Good luck findin' the profit sometimes ye have to travel a bit as well.