I've uploaded a save with a destroyer that will not be promoted to Combat VI.

http://forums.civfanatics.com/attachment.php?attachmentid=185943&d=1218842429

I'm using BTS 3.17, with Bhruic's unofficial patch and the BUG mod. The unofficial patch is from June 27, 2008. I don't think this is an issue with the patch, though. (I've got another issue from this same save, that I think has more to do with the unofficial patch. Thread is here: http://forums.civfanatics.com/showthread.php?t=287387).

According to the Civilopedia, Naval units should promote to Combat VI.

Let me know if anyone needs more information.

Argh. I'm an idiot. I guess Combat VI is only available when you attach a Great General.