Bloodrast, yes there were a lot of men on it which was part of the reason for hitting it with a large force. (That and the fact that you need a huge advantage ship-to-ship to beat the AI in naval on "very hard.") Their crew took a lot of casualties, but the army took none. I hit with a smaller force the next time (3 or 4 boats) and it went to the bottom easily with few casualties on my side. The "feel" was the same as if there had been no army on board. I knew I needed to kill this boat to prevent them from taking Sicily.

So far I'm finding it too expensive to conduct naval warfare. Part of that might be that I've got so many enemies that I run out of boats too quickly.