Great article, Robert. I've found myself in the same situations! I had finally found myself ready to target the bigger ships when lo' and behold, just ahead of me were two Man-o-Wars duking it out. I kept just out of reach to watch the outcome with my spyglass and as I saw the battle winding down I sent my mortars straight into the Spanish vessel, swinging in for the killing blow (and all that booty). I had managed to avoid clipping the English vessel so they paid me no head, but after taking the galleon I did pause for a moment to train my broadside cannons on the other ship. I was quite tempted to trounce the oblivious ship, but instead said to the TV set, "You're welcome, and safe sailing," before heading away. Not 30 minutes later I saw a wounded English Man-o-War just sitting in the open seas by itself... they were not so lucky.
I may be in the minority, but I actually think that AC4's land stealth is greatly improved from the previous title, namely that they managed to make the buildings closer together with more natural hiding spots and corners, as well as making trees a viable stealth option. The ocelots will never see me coming ^.^