Depends. 26 of those Green boats (which is one unit of them on large) will beat just about every pirate stack hands down. And most other navies as well.

The one really good thing about those elite units is, that provided you use them to support your campaigns rather than fighting them all by themselves, they will last a long way. I've had one squad of Pheraspides + one squad of Hypaspistai within an army which has taken at least 15 towns. And the squads still were on fighting strength.